He is a lone human from the City of the Dead, born with vague memories of a past life and raised by a group of undead until the age of adulthood, he swore an oath to Gracefeel and leaves the city in search for adventure and civilisation while he restores the faith of his guardian deity.
He was brought to the City of the Dead as an infant by demons with the intention to be used as a sacrifice to break the seal placed upon the High King, however, he was found and taken in by a group of undead, these three undead were Blood, Mary, and Augustus.
William recalls that he had a previous life in Japan, he can only vaguely remember certain events and common knowledge, his previous life was lamentable which becomes his determination to lead a proper life in his current one.
(Source: Saihate no Paladin Wiki)
