

It's easy enough to get Billy in a regular playthrough, since you wind up accidentally doing target actions all the time as is. You can only fight one on each Arcade playthrough. Beating them will unlock them and get you an achievement for each of them. If you did it correctly, after the 3rd stage, you'll get the "HERE COMES A NEW CHALLENGER" prompt and either Billy (if you did 2) or Saiki (if you did 5) will face you on their own. It might depend on the time but otherwise they generally work most of the time. That said, the requirement is pretty flaky as I've got reports of people meeting the requirements for one of them and either getting the other or none at all. Just don't overdo it and accidentally pull of a 3rd one if you're after Billy. Then on the next opponent, do 2 target actions again and repeat.

What I mean by "each round" is, for example when you start Arcade mode on the very first match, perform 2 target actions, then beat the opponent. Target Actions are the bits of text telling you to do such-and-such like "Hit the opponent with a DESPERATION MOVE" To unlock them both, you must play Arcade mode and do a certain number of Target Actions each round. The main two unlockables for this game are Billy Kane and Human Saiki.
