Hannah Murray ( born July , 1989 ) is an English actress . She is best known for playing the role of Cassie , a gentle , ` spacey ' teen girl with an apparent eating disorder and unstated mental illness in E4 's Drama\/Comedy , Skins . Hannah Murray ( born 1 July 1989 ) is an English actress , best known for playing Cassie Ainsworth in the E4 teen drama Skins from 2007 to 2008 .