Kei Asagiri

A not-so normal high school student. A boy who has had part of his soul ripped apart and sent into the Netherworld. This allows him to use the mysterious power of black flames stronger than any other magic, but in exchange for the lost part of his soul which is constantly being eaten by the denizens of the other side.

When he uses his powers, black scales similar to a lizard appeared on his body. The numbers that appear are proportional to the amount of the soul that's being devoured. The more he uses it, the faster his soul will be eaten. If he overuses it, he will turn into something inhuman in both body and mind. Kei states that his body is that of a monster, but he still has human reasoning.
(Source: Baka-Tsuki)