You're of course welcome to your opinion, but Denise absolutely has the physique of a young adult, and what you call mentally immature, others might say is the result of someone who probably already experienced the world in quite a different way even before certain events caused her to withdraw into herself and create a place for herself without sharp edges.
But that's neither here nor there, at the heart of it is the matter that you enjoy one character but not the other and that is quite alright.
But their story lines together are so intertwined that I can't split them apart; they are constantly playing off each other in their ramping up of interactions with the MC and each other, and splitting them up would mean having to create three completely separate storylines (for their combined paths and each separate path). I already have my hands full just creating one game, I'm not about to create three at once.
So if you truly feel that way, there's not much to say than thanks for giving it a try (and I mean that sincerely) and better luck next time.