Pietà 

A story about two classmates who become friends, Sahoko and Rio. Rio is depressed and self-abusive and isn't loved by her "family" (which consists of her father, stepmother, and stepbrother), but Sahoko seems to be the bright spot in her life. Sahoko also seems to have a mysterious past and doesn't seem to have a good relationship with her parents; she lives with her aunt. It's also hinted at that Sahoko and Rio have met once before, when they were much younger... (Source: Lililicious) Note: includes 1 extra chapter