I know that Ocean likes to spoon feed information, but how much further Nika's and Nami's, "pushing boundaries" , went is one of the things that has puzzled me. When talking to Amber MC tells that he grow close to Nami after the Summers disappearance, how close was that ? Then I don't know how much of this talk I can trust as per Nika himself, he lied and we don't know about what. In 4.5 we learn that Nami did something, something bad that took a toll on Nika and he fears that her behavior now, to him is not genuine and is born out of guilty, that add to other hints in Ch5 suggest that Nami was more than just mean in high school. Finally in Ch5 Nika speaks how every time that he reach's out to Nami she pulls away leaving him in the dark, and that if she does this again he will never speak to her again ( maybe this is what he has not talked to Amber ? ), this implied that something more than just platonic love happened before the beginning of the game, but only in Ch4.5 we have inner thoughts of Nika seeing Nami as something more than her long time Roommate, and Nami's reaction followed the kiss is not of something that they did once and promised never to again, is of someone who screw around and panic when found out .Yeah, I'm guessing it didn't go far before Nika realised considering Nami's a virgin, but it's still something to consider. This might also be why he gets so flustered when she talks about her lady parts.
With all this I don't know what happened, one hand seems that the kiss in 4.5 was when Nika took action and Namis panic, but later it is said that Nika took action several times and Nami pulled away, I have some theories about all that, but Nikas comment don't makes sense if nothing happened before, but Namis reaction don't makes sense if someting has happened before