Hiroko Kaizuka (貝塚 ひろ子 Kaizuka Hiroko (manga), 貝塚 ひろこ Kaizuka Hiroko (anime)): Hiroko, nicknamed "Hiro" (ひろちゃん Hiro-chan) is an extremely intelligent girl, albeit quiet and modest, as well as Shiina's classmate and close friend. Additionally, she is also a dragon bearer, although this is not immediately revealed. A group of bullies consisting of Aki Honda, Miyoko Shito, Hiroka Takamura, and Mihaya Ozaki who torture her when she does not get low scores on exams; the manga reveals an occasion where bullies force Hiroko to ingest worms from a beaker repeatedly and a separate occasion where the same bullies rape Hiroko with a test tube. Additionally, Hiroko's parents scold her for not receiving better marks, unaware that their daughter's slipping grades are the price to pay for the bullies to leave her alone. She is able to only find solace in Shiina, however, she cannot approach Shiina about the bullying but must endure it until the day she snaps.
After experiencing the unfairness of modern society and being separated from Shiina due to her father's orders, Hiroko wishes that everything would disappear. This activates her shadow dragon, Oni, which she proceeds to use to commit a string of brutal murders including stabbing and dismembering Mihaya in a dark alley; crushing the skull of Aki Honda's brother (who Aki had a sexual relationship with, and who is only referred to as Aki's lover in the English manga) against a wall; 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".
Her Japanese voice actress is Sakura Nogawa.