Jason and Polly

"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

The relationship between Jason Blossom and Polly Cooper was passionate and romantic, however, it ended in tragedy. They became engaged and were expecting a child 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.

History
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.

Season 1
According to Alice, Polly was a shining star before she let Jason ruin her. Betty told Veronica about Polly and Jason's romance, that it meant everything to Polly, but nothing to Jason. Purportedly, the relationship quickly grew toxic, which led to their mother turning on Polly.

After the Semi-formal dance at Riverdale High School, Jason's body was found washed up at Sweetwater River with a bullet hole in his forehead.

An autopsy was conducted on Jason's corpse that revealed he had died on July 11th, a week after he had left Cheryl and was to meet with Polly, and in that week he had been held and tortured.

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 and Polly's name was listed next to Jason's, 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 find out more about Jason, Betty and Jughead attended his memorial at the Blossom Mansion so they could search his bedroom for clues. However, they were interrupted by Jason's grandmother Rose Blossom, who mistook Betty for her sister and revealed Polly and Jason's secret engagement to them. After the memorial, Betty told her father of what she'd learned, but he already knew of this. She also learned that the reason for the feud between the two families was because Jason's great-grandfather cheated the Coopers out of a fortune decades before and murdered her great-grandfather. Hal became angry and told Betty that Jason made Polly sick and that she couldn't come home until she wasn't sick anymore.

Continuing their investigation, Betty and Jughead unearthed Polly's whereabouts and went to visit her at the group home she'd been sent. There, Betty learned of Polly's pregnancy as well as what really happened leading up to Polly being locked away. However, during their talk, Betty realized that Polly had not been told of Jason's death and was forced to tell her the truth, devastating Polly. Sometime after Betty and Jughead left her, Polly broke the window in her room and escaped from the group home.

Later on, Betty and Jughead confirmed all that Polly had revealed to them when they found the car that Jason had stashed, still filled with Jason's belongings and the Serpent's drugs. However, while they left to notify Sheriff Keller of their discovery, an unknown someone set the car on fire.

Following her escape from the Sisters of Quiet Mercy group home, two searches were conducted to find her, one by Betty and Jughead and the other by the Blossoms, however, the Blossom's wished to find Polly as they suspected her of murdering Jason. While the searches were ultimately unsuccessful, Alice, in an attempt to staunch the rumors about Polly, held a press conference and announced to everyone that Polly was carrying Jason's child, much to the shock of the Blossoms.

Polly was eventually found by Betty, hidden in the attic of the Cooper house. Polly explained to Betty that she could not stay at the group home any longer, waiting for Jason now that she knew he wouldn't ever be coming for her. Since she couldn't live at home on account of her parents pressuring her to give up her baby, Polly wanted to go to the farm upstate like she and Jason had planned but she didn't have the money to make it. Through Cheryl, Betty sought help from Jason's parents who, at first, said all the right things but later Cheryl revealed that they only wanted Jason's baby and not Polly. With nowhere else to go, Polly went to stay with the Lodges at The Pembrooke.

Days later, Polly was interviewed by Sheriff Keller with Betty, Veronica and Hermione Lodge by her side. She explained her side of what had happened between herself and Jason; how they met, how their parents pressured them to break up, finding out she was pregnant and the subsequent engagement, as well as Jason's plan to fake his own death and for them to run away together. Sheriff Keller questioned her about the drugs found in Jason's car as well, and Polly was adamant that if was a one-time delivery for the Serpents. She soon became overwhelmed with grief as Jason's car had contained everything she'd had left of Jason, including her engagement ring he'd been keeping.

To try and mend the relationship between Polly and her parents, Betty and the Lodges threw Polly a baby shower, attended by both Alice and the Blossoms. Unfortunately, the shower ended abruptly as Alice and Penelope Blossom got into an argument during the gift portion of the evening. Upset, Polly claimed that the feud between the two families, the hatred for one another was what killed Jason. They were supposed to be together and their families wouldn't let that happened, costing 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 her away, because had she not done that, Polly would have met up with Jason, and he might still have been alive. Alice and Betty tried to convince Polly to return home, but Polly worried if her father, Hal, would try to make her another appointment to have an abortion, much to the surprise of Alice. In the end, despite Alice having kicked Hal out of their home for what he had done to Polly, she chose to live with the Blossoms at Thornhill.

Since moving in with the Blossoms, Polly stopped taking calls from Betty and Alice, much to their confusion. After many attempts by Betty to make contact, Polly revealed to Archie, during a banquet held by the Blossoms at the Belmont Lodge, that she only chose to live with the Blossoms because she suspected that they were involved in Jason's death somehow, and needed evidence to prove it. She was only refusing contact with her family to make her charade more convincing.

Quotes
"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

Trivia

 * 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.