Mitsuru Tsuwabuki is a fourth grader who attends Ohtori, debuting in episode 6. In this episode, Nanami believes someone is trying to kill her, but it turns out that Mitsuru is only putting Nanami in danger so he can come to her rescue and be noticed by her. He's her "boyfriend" for a short period of time, though she treats him more like a slave. He does not mind Nanami's mistreatment and longs to be her older brother, but also seems to have romantic interest in her, since in episode 18 he attempted to kiss her. His conflicting feelings for Nanami are probably due to his confusion about love, sex, and adulthood. This confusion and fruitless attempts to remedy it turn him into a Black Rose Duelist. In episode 24 it is revealed that he keeps a diary, where he's written out a number of plans to win Nanami's affection.