I have finished the game and I am only missing a bit of information about the girls (which probably is the reason why I ended up with 96% and not 100%) - other than that I collected every gift, dated every girl and beat the final boss as well. I also entered all bonus codes I am aware of. Still it seems that (in addition to those new bonus outfits) some are still locked for me. So is it even possible to unlock all bonus outfits?

Nevermind: I figured out that I'll have to date each girl at certain times as well to be able to unlock all unlockable costumes.