Not much is known about Cloak's early life, let alone his own personal information. His name is not known, and his very existence is not known among the average person.
At some point during his life, he as obsessed with heroes, as many people were and still are. He was also interested in being a hero himself, until he witnessed the death of his whole family (yes, including relatives). By this time, he had learned how to teleport, and he killed all of the murderers. This drove him to become a villain, as he wanted everyone to feel the same pain that he did as a child, seeing as it was unfair.
Cloak has taken a new identity and has become a well known villain among all others, and is merely an assassin. He does not cause trouble, as he does not want infamy among the average people of Boston. Instead, other villains or people who know of him come to him and request a task for him to do, which will only be accepted if it's related to killing anyone.