William Clayton is/was a master thief and black arts conduit who raised Flynn Ryder to be a thieving prodigy. He was extremely abusive, neglectful, and overbearing, and often threatened to sell the boy for profit. As a result, Flynn was raised with a paranoid mind and a cold outlook on trust, especially concerning people. He was taught to never turn down an offer if it could benefit him or his betters in some way (his betters at the time being Clayton in his perspective, more or less). He murdered Flynn's parents, Princess Lila and her consort, Kristoff. He also murdered Miguel, Milo's father. The first for sacrificial purposes and the second due to debts owed.
Clayton is not and never will be a normal human. He is actually a nagini, or a naga, a person of half-snake, half-human origins. He is capable of shifting forms and becoming a serpent, as well as becoming a monstrous half-serpent. He used these forms to terrify the boy (Flynn Ryder) into staying. His current whereabouts are unknown, as Flynn escaped him at sixteen and never looked back.