This has been bugging me for a while now so let me ask; why the hell are ms monday, rio and niki in the options?
I haven't played yaiba but monday never came across to me as a fighter (kinda goes against the game's tagline). The same goes with niki as well; neither of them are even playable (unlike Lisa and rio is an exception since she's a guest character) or have shown any sign of being able to fight (unlike Irene) so it seems like a real stretch to even consider adding them.
Out of all the existing girls that aren't playable, Irene is the most obvious choice to become playable (I honestly believe its inevitable, either as the dlc character or in doa6) so I'd narrow it down to either her or a new girl.