Hiroko Kaizuka

Hiroko "Hiro-Chan" Takamura was a friend of Shiina's who was in the same middle school class as her and the partner of a Shadow Dragon named Oni. She was bullied by Aki Honda and her group of friends (Mihaya Ozaki, Hiroka Takamura, and Miyoko Shito) who were jealous of her high grades. At the same time, Hiro's strict parents demanded perfect grades from her, unaware their daughter's slipping grades are the price to pay for the bullies to leave her alone. She was able to find solace only in her friendship with Shiina Tamai. Eventually, the unfairness of her world made Hiro snap after her strict father ordered her stay away from the more carefree Shiina, who he thought was the cause for his daughter's falling grades. She wished everything would disappear, a call answered by her Shadow Dragon, who proceeds to commit a string of murders on her orders. Oni stabs and dismembers Mihaya in a dark alley, curshing the skull of Aki Honda's brother (who Aki had a sexual relationship with); forcing worms down Aki's throat, then using a talon to rape her and tear open her abdomen before ripping her body in half; and throwing Hiroka out of a classroom window before the eyes of the rest of the class. Additionally, Oni severely maims Miyoko by ripping her lower leg off and breaking several of her ribs and then scars Shiina's left hand when she tries to fend Oni off.

When Shiina later confronts Hiroko at her home, she is unable to talk her out of her madness. Hiroko tells her that she has decided to stop enduring and that she still has one more person to kill, Shiina's father, who is held hostage by Oni. Seeing Shunji as an obstacle to her and Shiina's friendship and believing that Shiina would be better off without her father, Hiroko wills Oni to kill him as well. This sends Shiina flying into a rage, Hiroko chides Shiina's cowardice, her last words to Shiina are, "You can't save everyone with kindness".