The Atomic County
Marissa in the Atomic County universe is CosmoLass (Cosmo Girl initially) and sheâs still alive. In this universe she dates Kid Chino (Ryan Atwood). She possesses a magic bottle (that gives her superpowers) and she has a car named Cosmobile. In the animated series she has a lot of relationships, Battle Axe, Johnny tears, All American Boy and the Mentalist. Her archenemy is Mistress Widow, in the 7 episode she discovers that her mother is Mistress Widow and that one of her allies and ex-boyfriend (All American Boy) has an affair with her.
The O.C.
Marissa is daughter of Jimmy and Julie Cooper, sister of Kaitlin Cooper and her best friend is Summer Roberts and she is the on-and-off girlfriend of Ryan Atwood.
Season 1
Marissa is the rich socialite of Newport Beach. Her family was falling apart her father was caught committing fraud, her mother and her younger sister spent all the money.
When she discovers that her father and her mother were divorcing she was in Tijuana and Ryan, Summer and Seth took her to a bar, when she arrives to the bar she discovers that her boyfriend, Luke, was cheating on her with one of her friends. She runs away and she abused drugs and alcohol and her friends found her unconscious.
Her relationship with Ryan was always being interrupted by Luke or Oliver Trask (heâs mentally unstable and he hides an obsession with Marissa).
Her mother was romantically involved with Caleb Nichol, she discovered that when her mother had a brief breakup with Caleb she had a secret affair with her ex-boyfriend Luke.
Ryan and Marissa ended their relationship because Ryan went to Chino.
Season 2
Marissa struggled with alcohol and depression and she started to date another guy, DJ, when Marissa broke up with DJ, she and Ryan agreed to be friends.
Marissa had a lesbian relationship with Alex Kelly, Sethâs ex-girlfriend.
When Trey Atwood was released from prison he tried to rape Marissa, Ryan confronted Trey with the facts  and they got into a fight and Marissa picked up a Gun and shot Trey to save Ryanâs life.
Season 3
Marissa was expelled from her school to a public school, Marissa made a new friend, Johnny Harper, and he developed a crush on her but she was with Ryan and she wrote to him a letter saying that, after reading the letter Johnny drank too much and fell off a cliff. After that Marissa and Ryan broke up and she fell in love with Volchock .
In the end of the season she died in a car accident caused by Volchock.
Season 4
After her death, she still is a big part in the lives of Summer, Ryan and Julie.