Yakusai is an extremely cunning demon, whose pleasure lies in roaming his world terrorizing weaker demons, simply for fun. His personality is rife with a superiority complex that borders on the obsessive, feeding his ego at every opportunity. Being a sadistic demon, he derives pleasure from seeing other demons suffer. He has a rivalry with one of Mitsuki's seraphim, named Aster, he is always being defeated by Aster, and seeks to defeat Aster to prove that he is stronger.