Susan is a character in Perks Of Being Wallflower. She was priorly a friend of Charlie's in middle school. In the film she is portrayed by Julia Garner.
Susan dated Charlie's friend Michael and was described by Charlie as an 'old friend'. Yet when Charlie tried to sit with her at lunch on his first day of school she refused him the seat. It is also stated that she used to be a lot different and after her boyfriend's suicide she used her sexuality to try and help cope, and no longer associates with Charlie, since all of the conversations they tend to have result back to Michael.
Dated Michael in the 8th grade and was distraught when he died.
Used to be friends with Charlie through Michael but she iced him out in high school.