Even though we were twins, I used to demand I have my own birthday party. Until one year, out of the blue, Jason convinced me we had to combine them into one. It wasn't until years later, I found out why. It was because no one wanted to come to mine. And Jason didn't want me to know. He protected me. Every single day. I wish, that day at the river, I had protected him. I'm so sorry, Jay-Jay. We failed you. All of us.
Jason was the son of Clifford and Penelope Blossom, and the twin brother of Cheryl Blossom. He was captain of the school water polo team and a member of the Riverdale Bulldogs. He supposedly died under mysterious circumstances when he fell from a boat into Sweetwater River on July 4th, prior to the start of the series. But as it turns out, he died a week later around July 11th when he was shot. He is also the late father of Polly's soon to be born twin babies.
Jason grew up in Riverdale, alongside his twin sister, Cheryl Blossom. Prior to the start of the series, he was in a relationship with Betty Cooper's sister, Polly. According to Betty, he meant the world to her but she meant nothing to him. The relationship ended badly, Cheryl speculated that Jason was likely the reason that Polly suffered a nervous breakdown and was sent to live in a group-home.
Jason disappeared under mysterious circumstances on July 4th. That morning, he and his sister drove out to Sweetwater River together at dawn. They held hands and walked towards the water's edge. Cheryl asked her brother if he was scared, to which he gave a slight smile and shook his head. The pair rowed a boat. According to Jughead, that was the last certain thing anyone knew, as Boy Scouts stumbled upon a distraught and soaked Cheryl huddled on the rocks near the water. She claimed she dropped her glove in the water and he tried to retrieve it for her, tipping the boat over in the process, thus drowning. However, Moose and Kevin later discovered his body, which washed up on shore, while at the lake together. They noticed a bullet wound in Jason's forehead, revealing that his death was not an accident.
Who Killed Jason?
Led by Sheriff Keller, the police, opened up an investigation into Jason's death and carried out an autopsy on the body. Somehow, the results led them to Cheryl, who went with them willingly, claiming that she expected that they'd find out she was guilty. It emerged that Jason did not, in fact, die on July 4th, as previously thought, but a week later. 
When questioning Cheryl, she said that Jason wanted to leave Riverdale and never come back. She helped him come up with the plan at Sweetwater River so that their parents wouldn't go looking for him. They made all the way to Greendale in the boat and then Jason left, saying he would contact her in a month, but he never did.
Afterlife with Jason
While Cheryl slept in his bed, Jason appeared in her nightmare as a ghostly figure. Like a Gothic heroine, Cheryl slowly crept down the steps of her family mansion. She entered the main room, where Jason's coffin laid, only to discover that he was not in it, but the roofing of said coffin was riddled with bloody scratch marks. As she turned around, Jason reached out for her throat, Cheryl let out an ear piercing scream in response. Of course, it would soon turn out to be a dream.
During his memorial, Cheryl appeared unexpectedly, wearing a white dress, the same dress she wore on that fateful morning of July 4th, as she held an emotional speech about her brother. Later on, after the memorial, Betty and Jughead discovered from his grandmother, Rose Blossom, that he had intended to marry Polly. 
Romeo & Juliet
According to Polly, Jason's parents forced him to break up with her, they didn't approve of the relationship. But when Jason learned that Polly was pregnant with his child, he was extremely happy. They had planned to run away, start their family in a new and beautiful place. They were going to meet on the other side of Sweetwater River, on July 4th, but things did not go as planned. It was later revealed that Jason's car was left on the side of the highway of route 40, when Betty and Jughead went to check it out, the vehicle was there just as Polly said, inside, they discovered Jason's varsity jacket and drugs. After realizing that they had stumbled upon evidence, Betty and Jughead call the authorities, but by the time they got there, the vehicle was up in flames.
Jason appeared during a dream sequence of Jughead's, best described as some kind of idealized, make-believe version of the American dream. He and Polly sat across the table from Jughead, Betty and Veronica. Hal and Alice Cooper were there as well. They appeared to be one big happy family. 
According to Polly, what started out as a casual conversation blossomed into an epic-forbidden romance. She was a Vixen and he was a Bulldog. They had a wonderful relationship until their family's drama got into the mix, which tore the couple apart, as Jason's parents had forced him to break up with Polly. However, the break up was short lived as Polly revealed that she was pregnant with his child. This led to a secret engagement with Nana Rose's blessing, and her heirloom ring. At Pop's shoppe, they'd made plans to run away together and start a new life, but first, they needed cash. With that in mind, Jason made a one-time drug delivery deal for the Southside Serpents. They gave Jason an address upstate, where he was to deliver the drugs in exchange for money. However, the drugs never made it as Jason was murdered and the drugs went up in flames with his car. 
The Blossom Empire
Via flashback, Jason, Cheryl, Cliff and Penelope sipped upon the very maple syrup that their family founded Riverdale on. As the heir to the Blossom family empire, Jason was set to inherit the Blossom Maple Farms, however, his untimely demise sparked a turning point, which led many to wonder who would inherit the family business in his place. 
Last summer, Jason approached FP Jones at the Whyte Wyrm, a known hangout for the Southside Serpents. He went to the bar in search of a ride and money, so that he could skip town with his pregnant girlfriend. He and FP made a deal, in exchange for a vehicle and some cash, Jason would make a drug delivery for the Serpents. Unfortunately for Jason, FP eventually learned that he was the son of Cliff Blossom, leaving him to wonder why a rich kid like Jason would need the help of low life gang member. Jason explained that Cheryl was going to row him out to Sweetwater River on July 4th, tip the boat, and claim that he drowned.
While he did make it to the other side of the river unscathed, Jason was double-crossed by FP, who kidnapped him and tied him up in the basement of the Wyrm as Mustang taunted him. The plan consisted of them getting word to Jason's father that he was alive and well, but that his safe return would be reliant upon a large amount of money being paid forward by the Blossoms. Clifford visited him, but rather than wishing for his son back, he snatched Nana Rose's heirloom ring from around Jason's neck, before shooting him in the head.
Afterwards, FP and Joaquin DeSantos cleaned up the scene, stuffing him in the freezer, and later dumping his body at Sweetwater River.
Having gone through so much in the past few months, Cheryl had lost her will to live. After getting her affairs in order, she headed out to Sweetwater River, where she saw Jason last, and began pounding on the top layer of ice that had frozen over the water. As the ice gave away, Cheryl went under. It was there that she briefly hallucinated seeing Jason underwater with her. As a rotting corpse, with a hole in his head, he slowly reached out to Cheryl.
After Fred Andrews suffered a gun shot wound during a supposed robbery at Pop's Chock'lit Shoppe, Fred had a number of odd dreams during his drift in and out of consciousness, one of which included Jason, who sat in the crowd, alongside his father, as Archie and Veronica's got married.
Jason was a moderately slim young man of medium stature, around 6' 1", with pale skin, red hair and steel blue eyes. Ironically, despite his leading position on the Riverdale Bulldogs, he was not an especially muscular individual. He was often seen wearing his blue and gold varsity jacket though. According to Veronica Lodge, Jason was "crazy handsome", Cheryl agreed, saying that he was the "handsomest".
As Jason was killed prior to the start of the series, not much information about his personality is known. He did bully Jughead at school from time to time. However, according to Cheryl, he was the best brother anyone could've asked for, and was a genuinely nice person who didn't deserve to be tortured and murdered. Jason was very protective of his loved ones, especially Cheryl. When they were younger, Cheryl insisted on having separate birthdays from Jason despite the fact that they were twins, until one year, he randomly recommended that they combine their birthdays into one. It was not until years later that Cheryl discovered the reason for this was that no one would come to her birthday. Jason protected her feelings whenever he thought it was needed.