He was a human-darkseer hybrid

experimented on, in his child years he had blue hair,
when he turned fifteen he dyed it green and  black.

He was found and raised by kagune in exchange for his loyalty. He has the power to control  electricity. Its shown that he views kagune   as a older brother. And while he wont let him know or see it, kagune see's shadow as a son and a younger sibling.

He shouldn't  be crossed however, despite  his somewhat, "friendly and kind personality" this is actually a ploy.

He is sadistic and cruel just like kagune, however  unlike his older brother figure he has shown mercy.