Mary Elizabeth is part of Patrick and Sam’s friend group, and she becomes Charlie’s first girlfriend. Mary Elizabeth is pretty and intelligent, and she has an alternative edge to her – she’s got a tattoo and a belly ring and is a practicing Buddhist. Mary Elizabeth runs her own zine – a small, DIY, artsy magazine – called Punk Rocky and manages the local performances of  The Rocky Horror Picture Show. She gravitates toward overseeing things, from events and projects to conversations. Mary Elizabeth is fiery – she enjoys styling herself in a way that might provoke people who are conservative or traditional, and she’s unfailingly blunt and honest, sometimes to the point where her comments can be a bit mean.

Mary Elizabeth begins to take a romantic liking to Charlie when he gives her a gift during their friend group’s Christmas celebration. Charlie gifts Mary Elizabeth money that will allow her to print the new edition of Punk Rocky in color. Charlie’s thoughtful gift shows that he’s been listening to Mary Elizabeth and that he knows what she cares about, and she begins to see him in a new light. Her crush on Charlie is later solidified when she sees him perform as the sexy and mostly naked character Rocky in Rocky Horror Picture Show, and she asks him to the Sadie Hawkins dance. Mary Elizabeth’s attentions toward Charlie make him feel happy, and he is genuinely attracted to her, but it’s clear that he still has stronger feelings for Sam despite beginning a relationship with Mary Elizabeth.

Mary Elizabeth is nice to Charlie throughout their brief relationship, often calling him on the phone to speak for hours. Charlie doesn’t understand the point of this constant communication, but it’s important to his growth that he experiences the little absurdities of high school relationships. Charlie also has his first successful romantic and sexual experiences with Mary Elizabeth, which include making out and touching her breasts. However, Mary Elizabeth has a habit of dominating conversations and only speaking about herself, which exasperates Charlie. He also doesn’t feel completely emotionally connected to Mary Elizabeth compared to his deep friendships with Patrick and Sam. For instance, it upsets him when Mary Elizabeth buys him a new copy of a poetry book rather than gifting him something from her own collection, as he feels it isn’t a meaningful gift. For Mary Elizabeth’s part, the gift is certainly thoughtful, but Charlie’s reaction shows that the two simply aren’t compatible. Meanwhile, Charlie always feels deeply connected to Sam and Patrick’s gifts, showing that there’s a greater understanding between them in comparison. Eventually, Charlie accidentally ends his relationship with Mary Elizabeth by kissing Sam during a game of truth or dare. Charlie feels terrible about his mistake and understands that he’s hurt Mary Elizabeth’s feelings. Eventually, Mary Elizabeth finds a new boyfriend, a college student named Peter, who better suits her. Peter can match Mary Elizabeth’s assertive energy and participate in discussions with her. On the other hand, gentle and quiet Charlie didn’t know how to handle Mary Elizabeth’s dominating personality. While it’s clear that the two weren’t meant to last, Mary Elizabeth plays an important role in Charlie's journey toward being an active participant in his own life.