Katie is Emily Fitch's twin sister, who she is older than by a few minutes. While they were young, they developed their own cant-style language that only they understand.
Katie is initially presented as being very manipulative and very outspoken, seemingly using people for her own gain. However as the series progresses, Katie begins to show a different side of herself; she is actually very insecure and afraid of being alone, thus she tries to cover this up by acting nasty towards everyone and attempting to control every aspect of her sister's life.
During most of the third series, she is not close to anyone in the gang as her quest for popularity made her isolate herself. She is especially cold to Effy, Emily, and Naomi. She begins sleeping with Freddie, partly because she likes him, but partly because she wants to get back at Effy and be the gang's new queen bee. She seems shaken by her sister Emily's homosexuality, but eventually learns to accept it.
During the fourth series; Katie reconnects with her sister, becomes close friends with Effy and Pandora, and establishes friendships with Thomas, Freddie and even Naomi. She has become a much nicer person, and she and Naomi are much more friendly than they were in the previous series. Although she has become a better person, she still has kept her outspoken personality, most vocally standing up for Kanye West during the 2009 MTV Video Music Awards Controversy
Katie's profile on E4's official Skins website.