I feel like they could just do away with half the suspicion about how real their relationship is by telling a half lie. That they are childhood friends, but thought it was too cliche and embarrassing and wanted to hide it. That way they can keep "pretending" to date and don't have to hide that they know everything about each other.
Damn, it's been a while since I read this, I forgot a few things about this story, why are they trying to hide chemistry again?
Also, did she like him since the beginning or did she develop feelings after the fake relationship