A first year high-school student who appears to be gentle and kind. Despite his quiet disposition, he is usually quick to voice his discomfort and concerns when it comes to the operation of JIGOKUGRAM. While he expressed wariness towards Iustus, he seems to get along well with the remaining prisoners, especially those who are younger than him. Mayumi shows a strong willingness to support and connect with his fellow prisoners, but tends to put himself before others, likely due to his early independence.

Calm and respectful, but has bluntness underneath.

Voice Claims: Hijiri Koganei, Osamu Dazai, Isogai Yuuma