Sister's Boyfriend, called Derek in the film, is a character in The Perks of Being a Wallflower. In the film, he is portrayed by Nicholas Braun.
In the film is refered to as "Ponytail Derek".
Charlie thinks that one day Derek will have regret having a ponytail.
In a relationship with Sister (Candace in film). Sister is very mean to him in the beginning of their relationship but that stops when he hits her across the face in front of Charlie. Charlie tells his English teacher about the incident and the teacher tells Sister's parents. Sister and he continue to date secretly. During the relationship, it was shown that she visits him frequently, even buying him Christmas presents. When Sister ends up pregnant, he breaks up with her. In the film, the pregnancy is only included in deleted scenes. And another spoiler he just wanted to knock her up because his friends dared him and said he wouldn't do it, but he did and won the bet.