J. Finn also known as T9581, was a human naval officer turned into a mutant.
Finn was married and had a son. He was a former friend of John Bishop, who attended his wedding.
After he was turned into a monster, the Purple Dragons stole weapons from Bishop, including T9581. At the Purple Dragons' lair, T9581 attacked Hun, and then Leonardo and Casey Jones. He then tried to retrieve his wife, but Bishop and his men found him and attempted to get him under control. An explosion occurred during the struggle, which they presumed killed the monster. However, T9581 is then seen swimming in the sea, remembering his wedding.
How he was mutated is never specified, but given he was friends with Bishop, and Bishop himself mentions "genetic research," it raises a troubling possibility he was a test subject of his former friend.