Mom and Dad hated that Jason and I were dating. They were thrilled when Jason dumped me. Jason's parents forced him to break up with me. Of course, they didn't approve of a Blossom boy dating a Cooper girl. But when I told him about the baby, our baby... Oh, he was so happy, Betty. We were gonna run away. Start our family in a beautiful place.
When Polly and Betty were little, Betty was afraid of the dark. Polly would comfort her little sister, but their mother would never allow her to stay with Betty overnight, so she would turn on Betty's night light, a light she still had.
When Polly was nine, she ran away from home after a huge fight with her mother, she disappeared for hours, and the entire neighborhood searched for her. They would later come to find out that Polly was in the attic the whole time.
She used to be in a relationship with Jason Blossom, but it didn't end well. While they were together, Jason "meant everything to Polly, but she meant nothing to him," according to Betty. Her parents were furious about their relationship and her mother refused to acknowledge her as her daughter. This drove her to a mental breakdown, and she had to move to a group home. Her parents blamed Jason for this, but Betty blamed their mother.
Polly was also a member of the River Vixens before she was put into the group home.
When Betty helped Veronica take revenge on Chuck Clayton, they found the playbook that he and his football team members would rate their 'conquests'. In the book Polly was listed next to Jason's name, her worth being a total of nine points. Only further proving how little Polly meant to Jason, in Betty's perspective.
While investigating Jason's murder, Betty learned of Jason's involvement in dealing drugs from Trev Brown, a former teammate of his, and went to her father and asked about Polly. Hal explained that Polly and Jason had had a fight, he was unsure what the fight was about, but Polly was left devastated as a result. Hal alleged that Polly subsequently attempted to commit suicide, and that was the real reason she was sent away, to prevent it from happening again.
Sisters of Quiet Mercy
After months of being separated, Polly received an unexpected visit from Betty at the Sisters of Quiet Mercy group-home. She hugged Betty, who almost instantly noticed that she was pregnant with Jason's child. Polly had her suspicions as to why Betty never came sooner, her parents had likely been telling Betty that she was on drugs or alcohol. However, they actually told Betty that she was sick, to the point where she tried to commit suicide, and in return, they told Polly that Betty didn't want to see her, which she already knew was a lie.
Hal and Alice hated that she and Jason were dating, so they were thrilled when Polly and Jason broke up, but, according to Polly, that was due to Jason's parents forcing him to. He was ecstatic upon learning that Polly was pregnant and they'd planned to run away and start a new life together. On July 4th, Polly had packed her bags, intent on meeting Jason on the other side of Sweetwater River, however, her parents caught wind of her plan to run away, and in response, they had her sent to the group home. She then asked if Betty had talked to Jason and if Betty relay a message, at that moment it was clear that Polly had no idea about Jason's murder. She had planned to go to the highway off route 40, where they'd stashed a car, take it, and meet Jason, so they could pick back up where they left off, unfortunately, Betty was forced to tell her the truth about Jason's untimely demise.
Not long after her reunion with Betty, Polly ran into her mother in the halls of Quiet Mercy. Alice had come to take Betty home. Polly confronted her mother on the lies she told regarding Jason. As tension grew between the two and Polly's behavior grew irretrievable, two orderlies pulled her away, though Betty ran over to Polly, grabbing her hand and promising Polly that's she'd get her out. As promised, Betty returned, but by then, Polly had already escaped through the broken window in her room, leaving only blood behind, smudged on the glass window.
Polly appeared in a dream sequence of Jughead's, which would best be described as some kind of idealized, make-believe version of the American dream. She and Jason sat across the table, from Jughead and Betty as they held hands, her mother and father were there as well, they appeared to be one big happy family.
Following her escape from the Sisters of Quiet Mercy group home, Polly hid in the attic of her house until it was safe. After a hunch led on by a discussion she previously had with Jughead, Betty found her. She told Betty that she couldn't stay at the group home, waiting for Jason when he was never going to show, they initially had a plan to move to a farm upstate, and raise the baby, Polly still wished to go there, but she didn't have the money to make it. She couldn't stay at the house, as their mother and father wanted her to give up the baby, which Polly refused to do, and she won't have her child raised in a place that it's not wanted. Before leaving, Betty asked that Polly give her a few days to find her a safe place in Riverdale, as she doesn't want to lose her sister again.
Following a meeting that Betty had with the Blossom, in which they promised to take care of Polly and her child, she agreed to meet them at Pop's Chock'lit Shoppe, where she waited. However, only Cheryl showed up, she came to warn them that her family had no intentions of protecting Polly, in fact, they wanted her out the picture. With no place to go, Veronica and Hermione insisted that she stay with them.
According to Polly, who told her story to Betty, Veronica, Hermione Lodge and Sheriff Keller, what started out as a casual conversation between her and Jason, flourished into an epic-forbidden romance. But for reasons that were still unclear, their family's tried to tear them apart, however, the break up was short lived as Polly soon discovered she was pregnant with Jason's child. They became secretly engaged with Nana Rose's blessing and her heirloom ring. She and Jason had made plans to run away in an effort to start a new life together, the last time she saw him was at Pop's Chock'lit Shoppe, where he told her about his plan to fake his own death, and meet up on the other side of Sweetwater River. Unfortunately, before that could happen, Polly was taken away to the Sisters of Quiet Mercy.
Although, before they could even consider leaving town together, they needed money, which resulted in a one-time drug deal between Jason and the Southside Serpents. He received an upstate address where he would deliver the drugs in exchange for cash but neither Jason nor the drugs made it that far as he was murdered and the drugs went up in flames, in his car on Route 40, along with everything Polly had with him, including the heirloom ring.
A couple days later, Polly was approached by Betty and Veronica with the idea that they'd throw her a Baby Shower. Initially hesitant, Polly agreed, stating that she wanted her mother there, as well as the Blossoms. The shower would be the perfect opportunity to prove to them that she wasn't an unfit mother, despite their claims that she was.
While her mother had been invited to the shower, it was clear that the chances of her actually attending the event were slim, which left Polly disappointed as Betty attempted to cheer her up. Polly announced that she wanted Betty to be her child's godmother, if anything ever happened to her, Betty was the only one she'd trust to raise her child.
That night, as the Baby Shower commenced, Polly reunited with several friends that she hadn't seen in months, such as Valerie Brown and Melody Valentine. Much to her surprise, Alice decided to attend the shower, they hugged after Alice expressed how happy she was that Polly and the baby were safe. Just then, the Blossoms arrived, Cheryl, Penelope and Nana Rose; they gifted Polly with an expensive stroller, and in return, Cheryl wanted to be named the child's godmother, unaware that Polly had earlier named Betty the godmother. Halfway through the shower, Nana Blossom performed some kind of ritual, in which she hovered a purple stoned necklace over Polly's hand, which she claimed could read the baby's aura and predict it's gender, and according to her, Polly was having twins, one of each.
The gift opening portion of the night had come, and Polly opened her mother's gift to find the old night light that she used as a child. Alice saved it, she knew how much Polly loved it, and how much it made her feel safe, so she wanted Polly's child to have it. Following the night light, Penelope insisted that Polly open her gift next, which happened to be a miniature rocking horse that belonged to great-grandfather Blossom. Polly then received an invitation from Penelope and Cheryl to live with them at Thornhill, which didn't go over well with Alice, this subsequently led to a brief argument between Alice and Penelope. Polly jumped to her feet, yelling "enough", as she was sick of their incessant bickering, it was their pointless hate and rivalry that eventually led to Jason's death, Polly claimed. They were supposed to be together, but neither Alice nor Penelope would allow them to share that joy of being in a relationship, which cost Jason his life.
After the shower, Alice apologized for her outburst, but that wasn't enough for Polly as she was still frustrated with her mother for sending away, because had she not done that, Polly would have met up with Jason, and he could still be alive. Polly explained that she was alone at the Sisters, but that was never Alice's intent, she then pleaded with Polly to return home, and if she did, her father would just have to accept that. But Polly questioned if he'd attempt to make another appointment for her. Unbeknownst to Alice, Hal had made an appointment for Polly before they sent her away, he said that he's more than willing to pay to fix her problem, as she shouldn't have to live with her mistake. After learning the next morning that her mother had no involvement in the appointment and that she had kicked their father out the house, Betty insisted that Polly return home with them. Despite being free and able to come back home to be with her family, Polly opted to live with the Blossoms at Thornhill. She explained that she was doing it for her child.
Infiltration of Thornhill
After moving in with the Blossoms, Polly began to ignore Betty's calls and emails, cutting off all ties to her sister. However, she was in attendance at the Blossom family's ceremonial Maple Tree tapping, where Archie confronted Polly about her failure to communicate with her family. She told him that Betty had nothing to worry about.
Later that week, during a banquet at the Belmont Lodge with the entire Blossom family and board of trustees, Polly interrupted Cliff's conversation with Archie to ask him to dance. During their dance, Polly revealed that she only chose to live with the Blossoms because she suspected that they were involved in Jason's death, and needed evidence to prove it. She ignored Betty in order to make the charade more convincing, and that's why she needed Betty to stop trying to contact her.
Before heading off to bed that night, Polly stopped by Cheryl's room to say goodnight, unaware that Cheryl would go on to scratch her face out of the photo that was taken at the Maple Tree tapping.
Still fully content in her mission to out the Blossoms in having some form of involvement in Jason's death, Polly made her way to the east wing of Thornhill, where Clifford and Penelope resided. Inside one of the rooms, a shelf housing a variety of red toupées caught Polly's eye. She was then spotted by Clifford, who in actuality had grey hair. Angry over her snooping, he slammed the door in Polly's face.
With this recent revelation in mind, Polly called Betty to inform her that she was only running as co-Homecoming Queen with Cheryl in an effort to keep her distracted. While Polly hadn't found anything, she remarked that the family was certainly hiding something. Unfortunately, her earlier deal with Betty, in which they agreed if Polly failed to find any evidence by Homecoming, she'd leave, left her on a strict schedule. Although, the only place left to look was Cliff and Penelope's bedroom, however, they protected it obsessively. She had to cut her call with Betty short as Penelope entered the room with Polly's milkshake, and a request that she bring an end to her explorations of the east wing.
The next day, as Cheryl did her makeup, Polly found herself in the perfect opportunity to further explore the mansion by convincing Cheryl to borrow Penelope's jewelry. all the while hoping she would find something that would connect them to Jason's murder. Polly wondered how many wigs Clifford actually had, to which Cheryl replied one for every mood. As Polly rummaged through the Blossom's belongings, Cheryl discovered the heirloom ring given to Jason by Nana Rose, the ring that he proposed to Polly with. She questioned how Cheryl's parents could have it when Jason was the last person known to have it in his possession, she then attempted to forcefully take the ring from Cheryl as there as no way Jason would ever give the ring up. However, Cheryl wasn't in a complying mood, grabbing Polly by the wrist and threatening her.
Later that night, Penelope came into her room, offering an explanation as to why they were in possession of the supposedly missing ring. According to Penelope, the last time Clifford saw Jason was the day he disowned the Blossom family, wanting nothing more to do with the name or lineage, including the ring, which he threw in Cliff's face as a final rebuke. Cliff then gave the ring to Penelope for safekeeping, but Polly was under the impression that the ring was lost, but "nothing's lost forever, everything comes back", Penelope said. Before she exited the room, Penelope insisted that Polly finish her milkshake, which unbeknownst to Polly was drugged with a relaxer, knocking her out for the night.
Returning to the Cooper Fold
Having missed Homecoming, Polly woke up the next morning to surprising news, a published article in The Register, which stated that FP Jones had been arrested for Jason's murder. Clifford gently touched her hand, telling Polly that he went to the station, and looked Jason's killer in his eyes. Finally, they would have peace, as would Jason, or so his parents claimed.
During the late hours of the night, loud knocking could be heard all throughout Thornhill. Those responsible for the late night racket was Polly's mother, father, and sister, coming to take her home after a startling revelation that she and Jason were relatives, third cousins to be exact. According to Alice, Hal's grandfather wasn't just the murder victim of Clifford's grandfather, they were brothers, thus making Polly a Blossom. Penelope stated that nothing could be more purely Blossom than Polly's babies, a comment that left Polly sickened to her stomach. Disgusted with this latest turn of events, Polly decided to leave with her family.
The next day, she and Betty laid in bed together, discussing their current predicament. Betty was thrilled that she had finally returned home, that she was safe, and that the babies were healthy. But everything else was like a nightmare. 
In the wake of Clifford Blossom's death and the recent revelation that he was responsible for Jason's death, the Cooper family resorted back to their routine life. Polly and Hal had returned home, but much to Betty's dismay, they were all smiling and simply avoiding all the damage the town had suffered, their family included as they weren't the perfectly normal family they pretended to be.
Later that day, Polly walked in on Betty and Veronica gossiping, an activity that she missed dearly, as well as being a River Vixen. Polly even missed going to school, however, her mother wasn't too fond of the idea, but what she didn't know, wouldn't hurt her, Betty said.
As planned, she accompanied Betty to school the following day. Unfortunately, their fun outing quickly turned bad as someone wrote "Go To Hell Serpent Slut" in pigs blood on Betty's locker, in retaliation to the article she published in FP Jones' defense. Sadly, their terrible day only seemed to worsen as Polly was interrogated by her mother on the events that had taken place earlier at school, to which she apologized to Betty for.
The following night, Polly attended the 75th Anniversary Jubilee with her parents as Betty gave a speech to the entire audience in direct relation to the town's current dilemma, in which they were facing a crossroads. They needed to face the reality of what they were because lying and keeping secrets would only damn the town, and what happened to Jason wouldn't be the last if they continued their ways. Riverdale must do better, Betty said, as Polly, Alice, Hal, and the rest of the audience gave Betty a standing ovation. 
The next day, she told her family that she was leaving Riverdale to live at the Farm. She had already called to make sure they still had room for her. What Farm, her mother asked. Polly replied that it was the very same farm that she and Jason were going to run off to before he was killed. Before any rash decisions were made, Hal suggested that they all sit down and talk about Polly's sudden urge to leave. Polly explained that her need to evacuate stemmed from the Black Hood, who was targeting sinners. Betty did not believe that Polly is a sinner, but Polly countered that she was a mother that was carrying her cousin's babies, and decided that leaving Riverdale was the best way to avoid being targeted. Alice strongly objected to this, saying that Polly would leave Riverdale "Over my dead body" Polly replied "It might be over your dead body, but it certainly won't be over mine or my babies" She then left the room, and Hal followed her to try and talk her down.
The following night, with her bags packed and a cab just outside the house, Polly said her goodbyes to Betty. Apparently, Betty was the only member of the family she was extending this courtesy to as neither of their parents knew that she was leaving. Betty questioned if this was truly how Polly wanted to leave things with their parents, going to the Farm without saying goodbye. Betty insisted that their mother was sorry. She's always sorry, Polly explained. Right up until she does the next thing that sets everyone off. Polly told Betty that she would be fine, before hugging her and promising to call once she was settled in.
Polly briefly appeared in Betty's anxiety induced nightmare, during which time she entered Betty's bedroom to wake her up from her sleep and tell her that Santa Claus was down stairs. Polly then joined their parents by the fireplace as Betty came down the steps. Moments later, Polly was dead, as were her mother and father. They had been murdered by the Black Hood who had disguised himself as Santa.
After giving birth to the twins at the Farm, Polly returned home to collect her belongings. While she was hoping to get in and out unnoticed, plans changed as Betty returned home from school. Her excitement in regard to Polly giving birth quickly turned into disappointment as Betty realized that their mother would be crushed upon learning that Polly gave birth without telling them. At the very least, Betty wanted to call their mother and alert her of Polly's return, but Polly requested that she refrain from such action. The leaders of the Farm, who Betty accused of being a cult, thought it best if Polly severed all ties from her family. Polly insisted that the Farm wasn't a cult. Furthermore, on the Farm, the twins, who were healthy and who Polly named Juniper and Dagwood, weren't subjected to any stigma. While their parents may have hurt Polly, Betty urged her to reconsider her decision to leave as her actions could be perceived as cruel, selfish and vindictive. Polly apologized as she prepared to exit the room with a box of her belongings. As she was nearing the doorway, Betty grabbed the nightlight sitting at the top of the box, claiming that it was hers, and that Polly would probably have no use for it anyways since the Farm likely doesn't have electricity.
Coming Back for the Blossom Will
Learning that her children's late grandfather, Clifford Blossom, left a secret will, Polly returned to Riverdale for the reading. She surprised her mother and Betty with her sudden return home. But imagine Poll'y surprise to meet their brother Chic, who she had no idea was staying at the house. After greeting Betty with a hug, Polly asked where their dad was at and if they would be attending the will-reading together. Which their mother had no knowledge of.
Later that day, Polly and Betty arrived at Thistlehouse with their dad for the will-reading. Also in attendance was Cheryl, Penelope, Nana Rose and Toni. In the will, Clifford's first declaration was that ownership of Thistlehouse remain with his mother, Nana Rose. With Penelope, Clifford left ownership of Thornhill, which Cheryl had burned to the ground. Not that anyone knew of this. Any and all remaining assets of Clifford's fortune was to be divided in half. The first half would be distributed equally to anyone in town who could prove with medical authentication that they were of Blossom blood. The will-reading was then interrupted by their mother who came to confront their father over him asking for a divorce for the sole purpose of cutting her out of the will. This led to a confrontation between their mother and Penelope Blossom. After their mother was escorted from the premises, the will reading continued. The second half of Clifford's assets were to go to his true heirs, Cheryl and the now deceased Jason. As Cheryl gave a speech, Clifford's twin brother, Claudius Blossom interrupted the will-reading.
After the will-reading, Polly, Betty and Chic discussed the blood test and how much money they would inherit through Clifford's will. Oddly enough, Chic refused to get his blood tested despite the fact that Polly explained that the Blossoms were their cousins. He claimed that he didn't want their blood money and needles triggered him.
The following night, Polly went on a mini shopping spree with the money she inherited. During which time, Polly told Betty that Chic was creepy and she didn't think that he liked her or the twins as she caught him staring at them a couple times. Betty agreed that Chic was indeed weird. It was part of the reason why she submitted a sample of his blood to the Blossoms' lab. Betty began to think that their father was right about Chic being dangerous, possibly not even their brother. And if he's not, Betty would be relieved despite their mother's heartbreak from learning this if it is the case. Polly and Betty returned home to find that Chic had taken Juniper and Dagwood for a walk along Sweetwater River. Just as Polly and Betty began to panic, Chic walked through the front door with the twins unharmed despite their suspicions.
The next morning, Polly came down to announce that she was leaving again. However, she was not returning to the farm, rather San Francisco, which came as no surprise to Chic, who had already told their mother that Polly wouldn't be staying for long. Chic told her that it was a nice city to raise kids and that she should send a note from her new address once she got there.
Polly is a very attractive teenage girl, with a slim figure, pale skin, blonde hair, and green eyes. Before she became pregnant with Jason Blossom's children and was taken away by the Sisters of Quiet Mercy, Polly was a member of the River Vixens cheerleading squad, meaning she would have likely been required to have a physically fit physique. As for her choice in style, despite Alice saying that Polly grew up too quickly, her choice in clothing does not reflect that statement.
Polly is a kindhearted but rebellious teen. She and Betty are much alike in that sense. However, Polly is far more susceptible to her mother's sway than Betty. According to Alice, Polly grew up too fast, and made a lot of mistakes along the way. And while she deeply loves her family, Betty especially, Polly refused to heed her parents warning of dating Jason Blossom. Her rebellious ways eventually resulted in her pregnancy and engagement to Jason. Beyond this, she is incredibly loyal. Her love for Jason was like no other as their relationship meant the world to her. Even in his death, Polly remained loyal by dedicating all her time and effort into finding his murderer. Although unsuccessful in recovering any incriminating evidence, Polly infiltrated the Blossom Family, hoping that she could find something that would tie them to his murder.
After giving birth, Polly was shown to be very protective of her babies. She acted in their best interest by choosing not to subject them to the hatred and toxicity that she was sure the town and even her own father would give her twins as they grew older. So, Polly decided to stay with the Farm. However, she later moved to the West Coast, setting in San Francisco, reaffirming her decision to live elsewhere.
As stated by Chuck Clayton, who claimed to have gotten his information from Jason, Polly was a bad girl, a prude by day but a freak in the sheets at night. However, this is not the only rumor that had been circulating that pertained to Polly. Whilst this has yet to be proven true, Polly has been referred to as being crazy on multiple occasion by multiple individuals. There was a brief moment at the Sisters of Quiet Mercy group home where Betty noticed Polly's erratic and less than sane behavior upon their reunion. Although, there has been no alarming behavior since.