Nobody gets used to pain. They can ignore it, and they can move past it, but nobody gets used to it.
Billy, for most of his life, has been abused. This is not something he'll ever admit to, nor would the word 'abused' ever cross his mind. There's a stigma with that. There's the mental image of awful families and deadbeat dads and alcohol, and that's never been the case. It's merely the fact that he was the particular favorite punching bag of his school's tougher crowd. And when he finally started trying to become what he felt he was meant to be--that is, a world-ruling supervillain--he then began to get frequent ass-kickings from LA's strongest superhero with muscles the size of his ego. And Billy, well, Billy isn't super in any sense of the word.
His mother worried whenever he came home with bruises. This was not as often as one might think. Most of the bullying involved being shoved into locker doors or being shoved into an open locker and being stuck in there until someone let him out. There was a lot of pushing and tripping involved. The jocks did not, of course, approve of a tattler, so he dealt with it as the other nerds and outcasts did--quietly.
His college days passed more or less without incident, a change of pace. He was generally either an ignored nobody or the guy everyone wanted to copy chem notes off of.
LA is a rough city, and it, like every other, has its good sides and bad. Billy always managed to skirt the edge between them.
The first time he had faced off against Captain Hammer, he was a one-hit knockout. He woke up about an hour later sprawled on the couch with his best friend/roomie/henchman Moist worrying over him.
Billy never learns how to fight. He's not good with his muscles, but he is good with his brain, and so he decides that using his natural ability will help him conquer any obstacle. This does not, however, bode well for his body.
He learns very quickly that long sleeves hide injuries very well, even if it's usually too hot in LA for such clothes.
He learns a lot of other things fast, as he usually does, along with the help of internet resources. Bruises are easy; one takes care of them even as a child. Lacerations are a different story. Fractured and broken bones another. He's able to get away with seeking hospital treatment only now and again for wounds or to check on them lest his frequent and persistent injuries with no good explanations cause the doctors and nurses to suspect something abnormal of the mild-mannered nobody. Some of what he learns is just by watching them.
He learns to set bones. He learns to stitch himself. He learns that a handful of assorted painkillers are necessary before popping a dislocated anything back into place. He learns that applying some makeup is a useful way of covering up the bruises on his face.
And it hurts. A lot. Pain is just as good a teacher as books are. It will always hurt, and there will always be pain, but he's also learned, potentially the most important of all, to deal with it. To make it hurt less. It bothers him, of course, and he still cries out in agony when severe pain is inflicted upon him, but pain has stopped being a big deal for him. It's something natural, something to be overcome, something that is so normal that he feels it is useless to make anything more of it than as what it is meant to be: an indicator of physical damage.