Ataru Mizorogi, the earnest homeroom teacher of Myojo Private School’s 10th Year Class Black, upholds his duties with unwavering dedication, oblivious to the classroom’s hidden reality as a lethal assassination arena. He attributes frequent student transfers to personal struggles, unaware of the academy’s orchestrated bloodsport. A Myojo Academy alumnus and biology instructor with three years’ experience, he shares an underclassman connection to Kaiba, a shadowed figure within the school’s administration.

Optimistic and deeply invested in his students’ well-being, Ataru adopts a protective older-brother persona. This facade cracks as the class dwindles, plunging him into bouts of melancholy and self-questioning, though he clings to positivity. Subtle inconsistencies in student disappearances occasionally prick his intuition, yet external forces swiftly mute these doubts. By the narrative’s end, he resigns to the class’s grim truth, channeling helplessness into steadfast teaching.

Standing tall with black hair and grey-eyed gaze, he favors casual attire—white shirts, blue jeans, and a loose tie—swapping to vibrant Hawaiian shirts during seaside excursions. Born in Shiga Prefecture, he relishes tennis matches and sushi while loathing parsley. His name, woven from kanji meaning "groove," "spine," "tree," and "place," carries narrative symbolism.

Spin-offs like *Koakuma no Riddle* delve into his past as a Myojo student. Exchanges with chairwoman Meichi Yuri underscore his earnestness, as she quells his insecurities about his teaching role. A civilian outsider to the assassination underworld, he lacks combat prowess or awareness of the carnage unfolding within his classroom.