Several characters can be used to level up Iron Fist Fullbody's special and will count towards leveling up the special as if it were a manual. See Hyper Straight for full listing. However, levelling up his special is a waste, because a, it is weak, and b, you want to evolve him ASAP and use other copies to socket the evolved version.
He evolves into a niche but useful unit (particularly for new players). So evolve him ASAP!!