Kai s a Villain and leader of the Eight Precepts of Death.

At the age where all quirks manifest, it seems as though Chisaki's dangerous and destructive "Overhaul" manifested and brutalized his parents. Alone in the world, he was adopted by the elderly head of the Shie Hassaikai, and from then on Chisaki held a deep-seated hatred of the genetic mutations which ruined his life. The psychological scars this left him with cause Chisaki to break out into hives whenever he is touched; his personality manifests this as repulsion towards germs, which bleeds into his views on quirks.

Chisaki has many dreams: to revert humanity back to a quirkless state, to repay the boss for saving his life, and to become head of the criminal underworld. Using Eri, the granddaughter of the boss, and her "Rewind," Chisaki believes her power and his might be the key to realizing these goals at last.

As Deku and co. tangled with Chisaki and his devastating power, the young head deteriorated the more he is impeded, touched, and backed into a corner, until by the end he'd become everything the Yakuza avoided to be: an inhuman, power-hungry force of nature.

(Source: Boku no Hero Academia)