Well to be fair, Amamiya puts up an act at school, and she actually follows through with what she's thinking as Futaba. It might be partially due to looks, but Kizaki's attitude towards her only changed when she started scolding him to take better care of himself. I'm surprised he didn't suspect anything when the phone call happened, but I guessed he didn't check out of respect to her privacy.
I want to see where the ambivalence comes into play!! When I first read the summary, I thought he'd spend more time chasing after Futaba before Amamiya would eventually fall for his kinder side.