BC

Billy Corrigan is an unintended manifestation of Originality™ and was created by Unk's repeated use of his cloning powers. Created from the combined amount of Originality™ left over from many of Unk's cloning attempts. This is explained by the Sixth Law of Originality™ and it is yet unknown if any other Unk based manifestations of Originality™ have yet been created.
Billy was manifested as an infant in 2013 but for reasons unknown, was hurtled back through time to 1967. Some prominent Unk experts theorize that his manifestation was somehow related to the release of One of a Kind / I am Stronger, but as of yet, there is no definitive proof of a connection.
Having lived a large portion of his life prior to the birth and unaware of Curtis Adams, Billy has been seemingly removed from "The Great Clash of Unks" but has demonstrated a considerable if less refined control of Originality™. It is unknown whether or how he has been in contact with Curtis Adams or any other clones, but given the breadth and intensity of The Great Clash, it seems likely that he has been.