I think Shiro is his sister and, as the guy with spiky hair said, she lost her memory.
Well I think we already saw his sister. That girl we saw from behind was one of the "Towa" candidates, looked more like his sister probably should, and hates men because they're "beasts." It's so on the nose that it's suspiciously on the nose... that's why it probably was her. This author is totally the guy that would be like "Haha, you thought it was too obvious but
that was the trap all along!" I could be wrong, man. Maybe the author is actually capable of following through with a normal misdirect instead of trying too hard? This is just the feeling I get offhand at the moment.