Effy is Tony Stonem's tearaway sister and one of the only people whom he truly cares about. Tony seems quite proud of her sneaking out of the house to go clubbing, having sex, and partying, and has helped her on more than one occasion to deceive their parents. It is apparent that she enjoys going out at night and partying, wearing skimpy, sexy outfits. She also rarely speaks in the first series, marking the episode Effy the first she speaks, where she describes how she feels towards others.
Tony sarcastically comments that she is participating in a sponsored silence. When asked by Michelle why she never speaks, she simply doesn't answer. On her blog she says that her parents had her see a counselor for withholding her speech. There is implication however, in the first episode and others, that she does speak to her parents only occasionally, although after Tony's accident she starts speaking again. Expressing her feelings of dislike to Tony, he snidely remarks to her that she "doesn't fool him".
Effy is one of few characters in the series who has broken the fourth wall, having looked straight at the camera and commenting or smirking in some episodes.
By the third season, she is considered to be the queen bee at Roundview, having shed her mute state and continuing to be best friends with Pandora Moon. She eventually catches the eyes of best friends James Cook, Freddie Mclair, and JJ Jones, where much of the season revolved around their complicated love square.
In the fourth season, Effy finally chooses Freddie, which causes Cook to lash out, eventually causing himself to be sent to prison. She experiences a subsequent breakdown and attempts to kill herself. Although she goes to therapy with John T. Foster, he secretly wants her for her own and tries to alienate her from all her loved ones, even killing Freddie to do so. By the end of the series, Effy reverts to her former self and re-connects with Cook.
On her official Skins page, Effy claims she is distantly related to the 17th century French aristocrat called Cecile DeLacroix, who was beheaded during the French Revolution. She also says her favourite smell is petrol, and describes a peculiar talent of being able to pick up objects with her toes. Her best friend is odd girl Pandora Moon, who despite their differences, "gives the detached and cool Effy a heart."
Effy's profile on E4's official Skins website.