Shelley is Stan Marsh's older sister.
Shelley Marsh wears a grayish-white shirt and pink trousers. She has long, brown hair and bangs. She has bad dental problems, and has to wear a brace around her head, giving her a very unappealing appearance. Because of that, she is usually scowling or frowning. She has a distinct adenoidal, slurred voice, where she breaths heavily through her mouth and nose, and has a lisp problem.
Shelley Marsh is filled with a lot of anger and hatred, mostly caused by the brace she has to wear around her head. Her favorite way to vent this anger and hate is to beat up her baby brother, Stan. The anger also comes from the fact she is not very good-looking, with or without the brace, and nobody likes her at school because of that. This issue is touched in the episode "Cat Orgy", where she confesses this to Cartman. This is the only instance where Shelley has been portrayed nice and shows a less tempered side of her.
Shelley was born in 1986 to Sharon and Randy Marsh. It is suggested that Shelley was not always angry, and that she only gained a temper after she got braces. The braces gave her a dowdy appearance, that gave her a low self esteem, causing her to pick on her brother Stan. No one belived Shelley was beating up Stan, due to the fact when parents were around she would act sweet and innocent. After the house was destroyed after a fight between the two, Randy and Sharon were planning on sending Stan to military camp. It was then that Shelley confessed she was beating him up, and it had been her fault. No one liked Shelley at school, which led her into a relationship with a twenty-two year old, Skylar, who did like her. He broke up with her though, after she wouldn't have sex him. This made her depressed and it took Eric Cartman, the boy she had been baby-sitting when Skylar broke up with her, to make her realize that she didn't need Skylar and he was a jerk. The two had been enemies, but soon became friends after getting revenge on Skylar.