Marcia Fadden is the Love Interest of Peter Gordon in IT and she is a rich kid from Broadway, a rich part of Derry and she is a rival of the Losers Club.
She is the only girl in Henry Bowers' team of bullies.
Marcia is a rich girl in Beverly's class along with other rich girls like Greta Bowie and Sally Mueller. Like these girls, Marcia believes she is much more beautiful and attractive than the others and she thinks because of this she will do better in life.
She apparently has a vacancy in the cheerleading squad, based on Beverly's comments about her.
Marcia seemingly feels outcast at school and because of this she only has one friend who is Peter Gordon. They are both neighbours on the same street and they presumably spend loads of time in romantic interludes. One day Marcia is on a date with Peter to the cinema and Peter notices Richie Tozier with Beverly and he begins mocking them and saying they should make out. Marcia, feeling protected with Peter, cowardly begins mocking Richie and Beverly as well and then Beverly tells them to piss off. Marcia is so shocked in the midst of her victory that she puts a hand to her breasts and recoils in horror but Peter protects her by putting his hand round Marcia's naked shoulders and threatening to meet Richie later to beat him up, even though it was Beverly who was mean. Marcia is led away protected by Peter into the movies.
Later, Marcia and Peter meet up with Henry Bowers and his gang and a scared Marcia tells Henry how she was "bullied" by the Losers. Henry and Peter vow to do what they can to protect Marcia and her beauty. Peter goes to extreme lengths throughout the Summer to protect Marcia from the Losers and anyone associated with them, even chasing down Mike Hanlon towards the Losers accompanied by Henry's gang.
However Peter is actually the first to flee Henry's gang because of his cowardice. Its presumed Marcia comforts him after the Rock Fight and they go off to celebrate by making love on a lake, and its presumed they are eaten.