I mean, it is a visual NOVEL, just one with more text. The fact that he talks too much is something I see as a positive, in contrast to all the other games where the MC gets thrown around with little say in the matter. He is suppose to be edgy and better-than-thou, he is a not very suttle copy of Sauron. As with him knowing things the player does not, how would one write a very knowledgable character? Everything he knows can't be told to the player, the game would be a hundred times longer if that were the case.
I don't think it's wierd, she is clearly an isolated and disliked girl who cannot find anyone who accepts her. Him already knowing her secrets took off the load of her needing to tell him about her, or her having such a secret the guy does not know about. She has found someone who accepts her, someone who shares hee darker instincts. That plus his corruption over her. Sure, it's not extremely realistic, but it is almost impossible to make such a game with fully realistic people, it's easier to make fun stories when they aren't.