In my view, Nami clearly has an ulterior motive when doing all the playful teasing: it's to try to get Nika to see her as a woman now (as in: potential partner material) rather than the goofy 'roommate'/best friend he's seen her as all his life. She loves him but can't come out with her feelings for fear of how he would react, so she has to tiptoe around it. Hence casually sending him pictures of her butt, taking her top off in front of him, asking if he thinks she's sexy, straddling him on a bed and so on.
While all of these would be considered the exact opposite of tiptoeing if done to another guy, that's not the case with Nika. Given the nature of their relationship, she assumes he won't read too much into it and will just chalk it up to her quirkiness and complete trust in him. That way she can show off her body, probe his feelings and reactions without raising suspicion and hopefully get his attention eventually.
Maybe that's also why she's so sensitive when he makes jokes about her breasts? Maybe she's insecure about it in general... or maybe it's because she feels like he's showing disapproval of her body?
Anyway, there are indications that she's been "working" on him like that for a while, since before the game started: she and Nika talk about how Nojiko thinks they have a "weird relationship".
Well... it's about to get a whole lot weirder if I have any say in it.