Katie is a classmate of Dave. When he's not fantasising about her, he's dreaming about being a superhero. When Dave's first outing as a vigilante ends in him sustaining critical injuries, everyone at school suspects that he moonlights as a gay prostitute. This works to Dave's advantage as Katie sees him as unthreatening enough to befriend.
Dave eventually gets the courage to tell Katie that he's Kick-Ass and not gay. This goes one of two ways depending on the source. In the comic, Katie chews him out for betraying her trust and has her muscular boyfriend beat him up. In the film, Dave and Katie become a couple and their relationship lasts up until the sequel, where Katie cheats on Dave after suspecting him of doing the same with Hit-Girl. She reappears briefly in the comics afterwards when the MotherF*cker and his gang take turns raping her until she's comatose .