Yumi Matou, a third-year Aoi Middle School student, oversees the school's server system with advanced IT and network security expertise. Her striking appearance includes a deep purple bob cut, violet eyes framed by oversized circular glasses, and a crisp blue-and-white uniform dress adorned with a white bow and accented with a yellow hairclip. Her physique, noted as 156 cm tall and 45-46 kg with 92-58-83 cm measurements, marks her as the most curvaceous female character in the series.

During the OVA arc, she capitalizes on Kei Ogata’s unauthorized server breach, orchestrating a blackmail scheme that coerces him into psychological submission. Her manipulative tactics reveal a sadistic streak, relishing his fear while masquerading as a top student with flawless academic standing. A past romantic involvement with Hideo Aoba ended under unclear circumstances.

Post-graduation, she enrolls in the same university as Kei and Shinichi Shiba, though her pursuits there remain undefined. The storyline forgoes redemption arcs, cementing her role as a calculating antagonist who preys on weaknesses for self-serving ends.