Charlie attended Oberlin College's Galactic Safe Sex Ball party as a sophomore where he was introduced to Marnie by his friend Elijah after Marnie had a bad reaction to marijuana brownies, and Charlie was the only person who comforted her. They began dating then. His best friend is Ray Ploshansky, who he occasionally does gigs with as a two-person band called Questionable Goods.
Charlie and Marnie have been together for four years by the Pilot, and he is perfectly happy in the relationship, though the sparks have gone out of it for Marnie.
After having sex, he get's into an argument with Marnie in Vagina Panic over how she's clearly not physically attracted to him.
He surprises Marnie with his newly shaved head in All Adventurous Women Do, which she finds repulsive until he says he did it in support of a co-worker whose going through cervical cancer.
In the episode Hannah's Diary, Ray finds Hannah's writing journal while he and Charlie are building a coffee table in the girl's apartment. Charlie later reads some of the passages out loud during a Questionable Goods gig about how Marnie should break up with him because she feel's so smothered in the relationship.
He breaks up with Marnie in the following episode despite her claiming those aren't her true emotions, but later takes her back after she goes to his apartment for the first time and promises to not take him for granted. Almost immediately after having make-up sex though, Marnie then ends the relationship.
He and Marnie run into each other at Jessa's surprise wedding in "She Did", but seem to be on much friendlier terms. He does however seem to still have feelings for Marnie when he watches her flirt with another guy.
Charlie and Audrey are still together, and they attend Hannah and Elijah's party in It's About Time. He later get's visited by an upset Marnie at his apartment, and he lets her sleep over.
Charlie brings Audrey to a dinner party thrown by Hannah in It's a Shame About Ray, and Audrey breaks up with him since he clearly still has feelings for Marnie. He follows Marnie to the roof and tries to make a move on her, but she rejects him. Hannah also yells at him when he tries to blame Marnie for what happened.
Marnie visits Chalrie in It's Back after hearing that he created a successful app and is running his own office space. His inspiration for the app also came from Marnie as it prevents you from contacting your ex's.
After Marnie embarrasses him at a company party in On All Fours, they fight and have sex in his office.
Now regularly hooking up in Together, Marnie becomes angry with Charlie about how the direction of their relationship. He admits that he still wants to be with her after she says she loves him, and they get back together. They're last seen smiling and walking together in Manhattan.
By the premiere episode Females Only, Charlie and Marnie have broken up after living together for several months. Adam mentions that he ran into him recently.
It's shown in She Said OK that Charlie posted an embarrassing music video of Marnie doing Edie Brickell’s “What I Am” on Youtube.
The story behind Charlie and Marnie's breakup is not fully known until Beach House, when Marnie explains that Charlie called her one day claiming that he wanted to propose to her, but that they needed to talk in person first. Charlie then came home and started packing his things, before telling Marnie that he did not love her and never had. Charlie left, and Marnie says she has not seen him since. Marnie also mentions that she heard his app business tanked, and Charlie is now working as a carpenter's apprentice.
Charlie re-appears in The Panic in Central Park when Marnie runs into him while walking down the street. He appears completely different, with an affected accent and a tougher demeanour. He explains that the reason he left Marnie and abandoned his old life was because his father had committed suicide, and he had not known how to handle it. Charlie ends up taking Marnie to an upscale party where it is revealed that he has become a drug dealer. Marnie seems accepting of this, and they go back to his dingy apartment. After having sex, Charlie asks Marnie to run away with him, and she accepts. But after finding heroin needles in Charlie's jeans, Marnie leaves the apartment without saying goodbye.