This is an Oumasai fanfic. (Kokichi Ouma x Shuichi Saihara) it has a setting and plot similar to Yandere Simulator.

Set in Pregame danganronpa v3. The new student Shuichi saihara a boy devoted to despair has an obsession with Ouma kokichi a timid introverted boy who also likes despair but is suicidal and self harms. Shuichi finds that he can't oppress his feelings about kokichi but when he finds out kokichi's being bullied and no one else is helping, he takes things into his own hands and maybe, the wrong way.