I thought Nicole was being set up as the "past" and once the MC settled down a bit at the hotel they would break up when he realized how much control of his life he'd given up to her. She steered him toward a law degree, helped him get through school, they moved in together (most likely because she told him it just made sense), etc. The whole "hometown always has rainy weather" and the hotel always has sunny weather seemed like a perfect metaphor for the past and future nonsense. Nicole dressed in grey clothes, sister dressed in bright clothes, etc.
Seemed natural. I may be wrong but at the beginning (and perhaps still?) Nicole wasn't even listed as a love interest in the game (I am probably misremembering though). In addition the entire idea of an open relationship where Nicole just sits at home and does nothing but the MC fucks every woman he wants, and, if she was somehow meant to be a "cuckqueen" watches (via the tablet?) and gets off on it felt ridiculous.
My main reason for not liking Nicole was because she seemed bored of sex with the MC, faked her orgasm, when the idea of a threesome came up she immediately thought of adding another man to the mix, and she didn't seem happy about life with the MC. And then when the open relationship idea was broached, with those points in mind, it isn't that unreasonable to think she wants to have sex with someone. Especially when she, in that episode, seems to be thinking about someone when she gets a call.
Regardless, I really wish the developer had written this out and didn't react to the audience. If people don't like the story you are telling they can go find another story to play. Now we have a situation where the MC has been at the hotel for a very short time and he goes from living with a long term girlfriend, to long distance relationship, to "he has a hall pass to fuck all the women he meets", to he is an asshole and breaks up with her (I'd be surprised if they have Nicole break up with him).