Jack is a very gentlemanly young Englishman and charismatic serial killer.
He had a very sad childhood where he was mistreated by his mother when he was very young for not finding his father and when he was very young he had to kill her
He sees only a few colors in people but when they die he sees a color that he appreciates so much, that's why he kills his victims, to see that color that he loves so much.
Now he is in Ragnarok fighting for humanity against the Gods who want to destroy them.