|“||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.||”|
|— Polly to Betty[src]|
The relationship between Jason Blossom and Polly Cooper was passionate and romantic. Nevertheless, their relationship ended in tragedy. They became engaged after learning they were expecting before Jason was mysteriously killed and Polly was sent to live in a group home.
It was later revealed that they were, in fact, third cousins. Polly's family had changed their surname to Cooper upon the death of her great-grandfather and they cut ties with the rest of the Blossoms, which Jason and Polly did not know about.
Polly has since given birth to their twins, who she named Juniper and Dagwood. Initially planning to raise them at the farm, just like she and Jason originally intended to do together, she has instead decided to raise them in San Fransciso.
Sometime prior to the start of the series, during the previous school year, Jason and Polly began dating. However, their respective parents opposed the relationship as the two families despised each other and actively tried to tear them apart. Jason ultimately caved to his parent's wishes, ending things with Polly, though the break-up was short-lived as Polly came to learn she was pregnant.
Upon being told he would soon be a father, Jason was happy and he and Polly resumed their relationship in secret. With his grandmother Rose's blessing as well as her heirloom ring, Jason proposed to Polly, and she accepted. They made plans to leave Riverdale and run away together, to start a new life on a farm upstate where they could raise their child. To finance their getaway, Jason approached the Southside Serpents and was instructed to make a one-time drug delivery to an address upstate in exchange for cash.
To ensure that his parents wouldn't come looking for him, Jason, with the help of his sister Cheryl, made a plan to fake his death, however, Jason refused to tell Cheryl about Polly or why he was leaving. Polly saw Jason for the last time at Pop's Chock'lit Shoppe, where he explained what would happen; he and Cheryl would go on to stage a boating accident at Sweetwater River, the morning of July 4th, and make it seem as if Jason had tragically drowned. In reality, Cheryl would leave him safe and sound on the far side of the river. Jason had a car stashed on the lost highway off of route 40, filled with his luggage as well as the drugs he was to deliver for the Serpents, that Polly was to use to meet up with Jason so they could begin their new life. On July 4th, before Polly could leave her house and put the plan into action, her mother got wind of it, as well as her pregnancy, and had Polly forcibly sent to the Sisters of Quiet Mercy, a group home for troubled youths. To explain Polly's absence, Alice and Hal Cooper told everyone, including Betty, that she break up with Jason had caused her to have a nervous breakdown and had been sent away to get help.
After Cheryl left Jason on the far side of Sweetwater River and staged the accident as planned, Jason's whereabouts became unknown until his body washed up on the same river.
|“||We were supposed to be together... And you didn’t let us have that, neither of you! And you know, it cost Jason his life.||”|
|— Polly to Alice and Penelope[src]|
|“||Such a shame. Your wedding was the last thing I was living for. I lost a grandson, but you... you've lost the love of your young life. Poor child||”|
|— Nana Rose to Betty, who she thinks is Polly[src]|
- In the football player's playbook used to keep scores of their “conquests”, Polly Cooper was listed beside Jason's name as: 9 (points), "shy reserved girl'.
- They both were unknowingly in an incestuous relationship, as they are third cousins, but didn't know it when they were together.