Pretty much guaranteed, which means almost, but not completely in this case. The near guarantee in this case is not specific to LiL at all, but is instead because of the genre of the game. Remember that this is a Denpa game, like Doki Doki Literature Club, as stated by Selebus, a core feature of which is a plot that gets dark and rarely goes back. I am most definitely not talking about Chika specifically, there's been something brewing in the shadows and this will affect the entire cast when whatever it is hits. As for how Chika can't see that she is not his only girl, she's naive, too naive for her own good. She thinks that he couldn't possibly do something like cheat on her and it could very easily come back to hurt her, badly.
Not many characters are showing signs yet and most of them don't suffer from any condition that could change that, but they could slip once whatever is brewing in the shadows, mostly hinted at through 'happy' scenes, comes out. The 'happy' scenes and the main events both serve as the main story of the game with the main events detailing how the cast is going about their lives and the 'happy' scenes hinting at the darker things yet to make an appearance.
