Don Tracy was a young orphan who got caught in a explosion meant for the Green Mask. The Green Mask took the boy to Professor Lascomb, who used his "vita-rays" to heal Don's injuries. Don decided after that to become Green Mask's sidekick, Domino.
Domino doesn't have super powers like Green Mask cause he was exposed to the right amount of vita-rays. But he was still good in a fight and was experienced with a boomerang.