Muzan, kokushibo, and douma are high school students and this three study in the 10th grade

muzan—He himself was calm, cold-blooded and rather short-tempered. He absolutely does not care about his peers, except for Douma and Kokushiba

Douma—presented as a cheerful and cunning person, he has no true emotions, he fakes them, he is quite loud, violent, and often disrupts lessons

Kokushibo—His yellow eyes are always filled with indifference, calmness and indifference