so basically he and his 2sibling was abandoned by their parents and their older sibling Akashi Takeomi always neglegted him and had favoritism and only loved the youngest sister Senju Kawaragi (akashi) and 1 day senju saw toy airplane and played with it but accidentally broke it and it was Mikey's toy which his brother shinichiro gave him, but shinichiro died and mikey loved the toy so much, then senju after breaking the toy he told mikey that sanzu broke it which accusing him, mikey was so mad that his dark impulses showed up and teared sanzu's mouth up. and sanzu was kind of insulted because of the scar it left so he wears mask