Billy is one of the most difficult and temperamental members of the ZOE team. He is the one who refused to acknowledge White Fang until the end.
He has the power of Gigantamax in him. Because of this, he was abducted from his parents at an early age and repeatedly experimented on as a biological weapon. His grief soon turned to unfathomable anger, and he would not give in to any training with his fearsome hatred.
Even after he was released from captivity, he did not believe in humans. The OTW was where he finally drifted off to. He fought many one-on-one battles with ZOE, who became his master, and was defeated each time. Over time, he became friends with her and Salty(Eiscue) and recognized them.
Billy can be intimidating, but he also has a caring personality. He is not the strongest of the OTW, but he's pretty good. Although rough, he is good at brainstorming with poison and electricity, and even OTW's strongest frame, White Fang or BUCK, will not be able to avoid heavy damage.

Incidentally, Billy had a setting where he had tattoos carved all over his body to remove scars from his past, and he made it his aesthetic. He may revive it somewhere in the future.