Mary Elizabeth is quite a pill, if you ask us. With a tattoo and belly button ring, she's a teenage American Buddhist, and she has a lot to say about the matter. She tells Charlie a bit about Zen and how "it makes you connected to everything in the world" (2.1.4). Basically, she's the opposite of Charlie, a guy who totally separates himself from reality.
Perfect match, right?
Apparently something in Charlie draws Mary Elizabeth in, because she asks him to the Sadie Hawkins dance. And this is after seeing him a skimpy gold bikini. Charlie does his typical Charlie thing (i.e., doing what Sam tells him to do and basically just smiling and nodding at everything Mary Elizabeth says), and she kind of takes over the relationship.
Charlie gets angry about this, but she really has to take charge. Someone has to do the talking, after all. Unfortunately, she exercises her influence a little too much, buying Charlie gifts and making him show them off. Charlie's kind of like a little trophy boyfriend, except that he's holding the trophy. So we guess he's more like her trophy-holding boyfriend—even worse, if you ask us.
Once Charlie gets totally fed up, he humiliates Mary Elizabeth by kissing Sam in front of all their friends. Well that fizzling relationship sure went out with a bang. Eventually she moves on with a guy named Peter, saying that she and Charlie's relationship was "too one-sided. She wanted a person who was more open to discussion and didn't need someone's permission to talk" (4.3.56).
She sure has Charlie pegged there, but at the same time, she kind of enabled that behavior. Hey, looks like passive-aggressiveness is one thing they have in common.
In the end, Mary Elizabeth does visit Charlie in the hospital, and she stays longer than all his other friends. That says a lot about her character—she is willing to forgive him and move on. She's willing to be his friend.