Well, I was pretty off the mark with my previous comment, but since the previous chapter hinted a "yakuza arc", I still think that the new girl is related to the yakuza somehow...
My bet still on she's the daughter of some yakuza boss and because of this she needed (or was forced) to learn martial arts to protect herself. Eventually becomes Akira's rival in tourments and lost a lot of times to her, that's why now she wants Kotaro, to finally have a "win" against Akira.
It's a theory, but I'm on board with this.
Ps: Sumika still the best girl.