From the HBO Official Site:
"Marnie is Hannah's roommate and best friend. Highly responsible and very put-together, she has a very adult job at a very adult art gallery and a serious, long-term boyfriend -- neither of which she can admit she might not love (or even like)." "She's 23 years old."
Marnie is arguably the most mature of the group. Her parents are divorced. She has a semi-friendly relationship with her mother, Evie. She met Hannah and Jessa as well as her long-time boyfriend (now ex) Charlie Dattolo at Oberlin College. At the start of the series, she works at a small gallery.
It is implied that Marnie suffers from an eating disorder which explains her very slim figure.
At the start of the season, Marnie's main struggle is breaking it off with Charlie, her boyfriend since college. She lives in an apartment with Hannah, though Charlie stays over more often than not.
In the premiere, Marnie is struggling with the fate of her relationship with Charlie. She describes his touch as that of a "weird uncle." At Jessa's welcome home dinner, Marnie becomes angry when Hannah and Jessa both arrive late. To make matters worse, Charlie invited his bandmate, Ray, and a random girl to the dinner. With her patience wearing thin, she attempts to confront Jessa, who announces that she is pregnant.
In Vagina Panic, Marnie schedules both Jessa's abortion and Hannah's STD test. At the clinic, Marnie begins to worry that she is unable to get pregnant herself, and claims she was born to be a mother. After Hannah comforts her, Hannah goes to have her test. Shoshanna then reveals that she is a virgin, and Marnie comforts her.
In All Adventurous Women Do, Marnie attends an art gala, where she meets Booth Jonathan, a prestigious and cocky artist. The two go outside together, where he predicts the two will eventually have sex. Marnie re-enters the gala, goes to the bathroom, and touches herself. Later that night, she comes home to the news that Hannah's college boyfriend is now gay. The girls laugh and dance together.
In Welcome to Bushwick a.k.a the Crackcident, Marnie attempts to reconcile with Charlie, but is interrupted by his new girlfriend, Audrey. With Marnie's emotions out of control, Jessa tries to make her feel better, taking her out on the town. They meet Thomas-John, who repulses Jessa. The two decide to go back to his apartment, but leave after he expects too much.
In the penultimate episode, Marnie is increasingly more irritated with Hannah, and the two have a falling out.
In the season finale, Marnie moves out of her and Hannah's shared apartment, and into Shoshanna's. She attends Jessa's surprise wedding, where she meets the best man, and decides to impulsively hook up with him, much to Charlie's dismay.
At the start of the season, Marnie's employer downsizes the gallery and she is let go. She attends Elijah and Hannah's house warming party, where she hooks up with Elijah. The two decided to keep it from Hannah.
In I Get Ideas, Marnie attempts to get another job at an art gallery, but is told she doesn't belong in the art world. She then gets a job as a hostess at a club.
In Bad Friend, Marnie again meets Booth Jonathan, who takes her from her job to his apartment. After displaying his art to her, the two have sex. Soon after, Hannah arrives and calls Marnie out on having sex with Elijah, which he just told her about. Hannah calls her a bad friend, but allows their friendship to continue.
In It's a Shame About Ray, Marnie goes to Hannah's celebration dinner. To her shock, Charlie and Audrey are there as well. Audrey antagonizes Marnie for still having feelings for Charlie, and she leaves. Charlie follows her out onto the rooftop, where he tries to make a move. She responds by announcing her supposed relationship with Booth. Charlie angrily leaves Marnie alone.
In Boys, Marnie hosts an event for Booth after Soojin quits. She mistakenly has begun thinking her and Booth are dating, which he corrects. After Booth has an emotional breakdown, Marnie decides to cut ties with him and leaves. She then calls Hannah, and the two have a conversation in which they pretend they are better without the other.
In It's Back, Marnie learns from Ray and Shoshanna, who she was soothing after Jessa abruptly ran away, that Charlie has created a very successful app inspired by their break-up. After viewing his office and financial situation, Marnie is distraught. Ray then convinces her to follow her own true passion, which she reveals is singing.
In the penultimate episode, Marnie performs a song for Charlie at a celebration party for his app. He talks to her in private and tells her to get her shit together. The two then have sex on his desk.
In the season finale, Marnie and Charlie have begun engaging in casual sex, much to her disbelief. After a heated brunch together, the two ultimately confess their love for one another and end the night walking the streets of New York.
At the start of the third season, it is revealed that Marnie and Charlie are no longer together and she is living with her mother. She also has begun working for Ray at Grumpy's. While Hannah, Adam, and Shoshanna venture to get Jessa from rehab, Marnie moves into her own apartment.
In She Said OK, Marnie discovers that Charlie has yet to remove an embarrassing Youtube video of her, and fights with the telephone assistant about removing the video. That night, she throws Hannah's 25th Birthday party, where she forces Hannah to recreate a duet they did together in college.
In Dead Inside, Marnie catches Ray and Hermie watching the Youtube video and quits her job at Grumpy's.
In the Only Child, Marnie looks to Ray to tell her everything that is wrong with her. Surprisingly, this ends in the two having sex. Her and Ray continue to see each other as friends, because they have nobody else to turn to.
In Beach House, Marnie invites the girls to her mother's friend's beach house where they will stay the weekend. Marnie intends for the weekend to be healing for the girls' friendships, but it ends in total disaster after Shoshanna has a meltdown and the girl's argue.
In Incidentals, Marnie goes to Ray's apartment only for him to end whatever they have going on. He claims he wants a real relationship. Marnie leaves to the hotel room Hannah is staying for an article, where she meets Desi, a fellow actor who is in the same play as Adam, who expresses interest in her and her singing abilities.
In I Saw You, Marnie has begun working at Soojin's art gallery. That night, Marnie and Desi perform a song with success and applauds. However, Marnie see's Desi and Clementine's relationship with her own eyes, and decides to sleep with Ray again, and Hannah catches the two having sex.
In the season finale, Marnie admits to Shoshanna that she and Ray had sex, which she reacts to very negatively. At the opening night of Major Barbara, Marnie gives Desi a guitar pick, and he kisses her. At first she is pleased, until Clementine confronts her. In the end, Marnie stands in the street spying on Clementine and Desi arguing.
Season Four Edit
At the start of the fourth season, Marnie and Desi are officially a folksong duo. They are also in a relationship, even though Desi still hasn't broken up with his girlfriend.
In Triggering, Marnie is only briefly seen in a video chat with Hannah.
In Female Author, Marnie and Desi attempt to land a record deal, but are told they have to meet the head of the studio first. Afterwards, the two fight about their lack of an actual relationship.
In Cubbies, Marnie shows Jessa and Shoshanna a song of her and Desi's, which neither of them particularly likes. After they mock her, she claims that the only one capable of a real opinion is Hannah, who she calls a "creative." Later, her and Desi are practicing together awkwardly. Marnie claims shes moved on. However, later that night, Desi shows up at Marnie's apartment and announces he's left Clementine. She consuls him, but learns he only dumped her because she was going to dump him. Though she seems hesitant, he performs oral sex on her.