Yumi Matō is a third-year student at Aoi Middle School entrusted with the school server's maintenance, leveraging her advanced IT and network security expertise. She possesses deep purple hair styled in a bob cut, violet eyes typically seen behind large circular glasses, and stands at 156.1 cm tall, weighing 45.8 kg with a 92-58-83 cm figure, distinguishing her as the series' most buxom female character.

At school, she projects the image of a model student with excellent grades and a respectable reputation, consistently wearing the standard blue dress uniform, a white bow, and a small yellow hairclip. Beneath this respectable facade lies a manipulative and sadistic nature. Discovering Kei Ogata hacking the server, she seizes the opportunity to blackmail him into servitude, openly delighting in his panic and despair. She exhibits sociopathic pleasure in dominating others, openly mocking a teacher as "stupid" for using an easily guessed password.

Her background includes a past romantic involvement with Hideo Aoba, evidenced by photos showing them together in casual attire and revealing swimwear. After middle school, she attends the same university as Kei and Shinichi.