Enamel

Enamel (エナメル質, Enameru-shitsu) was a Mysterious Being that was killed by Metal Bat before he became an S-Class hero.

Appearance
Enamel looks like a giant tooth with his name as a tattoo on his body.

Weapon
He is seen lying dead after being killed by Metal Bat.

Abilities and Powers
While it is unknown how strong he was, the fact Metal Bat went from a C-Class hero to an S-Class hero after killing Enamel implies that he was quite powerful.