Well that was... not entirely unexpected given the indications in conversation. Roxas just hadn't expected it to come with so little provoking. Caught off guard, he staggered with the blow.

It might have taken him a little longer to decide on striking back if not for the remark about feeling better already after the punch. Really, this wasn't Sora, in some ways. Therefore, retaliating in kind didn't feel quite as bad. If Sora remembered this exchange, he would probably feel like shit, but Roxas couldn't afford to raise any more suspicions. If fisticuffs and nastiness between their mirror selves was common, he'd have to go with it.

"Oh yeah? How does this feel?" 'Feel' was emphasized with a boot to the groin.