Niki acknowledges that you sleep with other girls, she's just hoping you pick her over them in the end.
Which given she's a side character I don't think she'll get a purity route so rip.
Im not sure if you completely skipped over it or not, but there's a whole ass event where Akira tells her that he's broken and can't love her like that yet, and she says its ok but "don't keep me waiting forever".
She's underage so it is still rape.
Also she's very clearly not got her correct wits about her, I don't think it goes out of the realm of common sense to say she is very much not capable of consenting at that time, which ergo: still rape.