Helen is Charlie's aunt and an unseen character in "The Perks of Being a Wallflower".
Aunt Helen suffered from depression because when she was a small child, she was abused by her fathers' friend and in her later years by her boyfriend. The abuse caused her to have psychological problems which lead to her to gravitate to the closest source of comfort which for her was Charlie, whom she sexually and systematically abused. She later died in a car accident on Charlie's birthday while driving to buy him a present.
Aunt Helen was sexually abused by her father's friend when she was young. The results of that led her to abuse other children like Charlie.
She took her sexual abuse out on Charlie, sexually molesting him and touching him while his sister was asleep, and telling him to be quiet was her way of telling him not to tell anybody about her touches. Charlie tries to forget the abuse but is constantly reminded of it, especially when he makes out with Sam before she leaves for Penn State.