Ayumu Shiiba

A high school student who always struggles with her tests but is still very intelligent. She is a student at Nishidate High School, or Nishikō. (The word for high school is kōtōgakkō, or kōkō for short, so the school name is often shortened to Nishikō, or "Nishi high.") After the dissolution of her friendship with Shii-chan at the very end of middle school, she has difficulties fitting into high school life and starts to become a cutter. She lives with her single middle-class mom and younger sister, Akane, in a company housing.

When Katsumi Sako begins to threaten her, Ayumu tells the whole school and her own mother what he is doing--only to be totally ignored and treated as a liar.
(Source: Wikipedia)