She's thin, she's blonde, she says "wow" a lot.Cassie is depicted as being eccentric and suffering from several mental disorders — most notably, anorexia nervosa — and multiple issues, including low self-esteem, suicidal ideation, and drug addiction, but is gentle-natured and friendly.

[1] Murray described her character as... ...very interesting because she has a lot of problems, and she's very troubled, and she's anorexic and completely lacking in self-esteem and self-belief, but along with that she's sort of quite smart.

The show's producers and writers described her centric episode in an interview with The Independent as being about "how she gets through her day without eating ... how she feels and what her tactics are", and campaigned to prevent any announcement of any counselling helplines during the episode's credits because they "didn't want a little preachy drama where everyone hugs at the end"; instead, the producers hoped that people would recognize their own faults in Cassie.

Where before she was bubbly, cheerful and unashamedly quirky, she is now a shadow of her former self, appearing tired and withdrawn, living in a shared flat in London and working as a waitress in a café to make ends meet.

She avoids social contact with her co-workers and flatmates, and refuses to take drugs when offered them (as opposed to her affinity towards them in the past), and secludes herself in her room.

Her friend Michelle Richardson (April Pearson) arranges for her to take Sid Jenkins' (Mike Bailey) virginity at a party.

Cassie's central episode develops her eating disorder; her sexually-stimulated parents are introduced, who are usually too busy having sex in order to pay attention to her issues.

She postpones her plans after discovering Sid loves her through a letter he wrote at the beginning of the episode, and the series ends with the pair holding hands on the same park bench Cassie attempted suicide on.

Frustrated at Cassie's disappearance and emotionally confused by his father's death, Sid begins a relationship with Michelle in the latter's central episode, after she finds him at the edge of a cliff, possibly to attempt suicide.

As a result of Sid's relationship with Michelle, Cassie becomes extremely promiscuous (with men and women) and mentally unstable: in "Chris", her actions result in her roommate Chris Miles (Joe Dempsie) losing his job as a junior property salesman and cause a significant amount of tension between him and his girlfriend Jal Fazer (Larissa Wilson) after Angie, Chris' ex-girlfriend and psychology teacher returns to Bristol; and in "Effy", she attributes her addiction to "mindless sex" to her failed relationship with Sid.

There, she meets a kind young Iowan named Adam (Stephen Michael Kane), with whom she forms a strong platonic friendship when he offers her a place to stay in his apartment.

The ultimate scene of the second series depicts Sid wandering through Times Square looking for her and ends with him turning to look inside the diner.