Rumi is a lonely primary school student who is openly despised by her peers. Hayato at first tried to resolve the situation through talking to her in front of her classmates, which instead attracted more hateful attention to her. After much discussion, Hachiman and company decided that the only way out is to destroy the trust between her peers, through having Hayato and Kakeru acting as bullies threatening Rumi's group-mates during treasure hunt and exposing their selfish nature to each other, thus preventing further organized acts of teasing on Rumi. However, in exchange, she would hardly have any friends after this. (Source:Wikipedia)
