I'd like to question the use of the tag kotobuki_seishirou. Sure, that's technically her real name, but she goes by the name Kotobuki Seiko, (pretty much) everyone calls her that, the dudes over at Wikipedia think it's her official name, and it gets WAY more Google results than Kotobuki Seishiro(u).
If nobody objects, I'd like to change the tag to kotobuki_seiko, as I feel it's her "official" name in the series.
I think the real name is preferred on Danbooru. For what it's worth, the official anime websites list him by real name, as do the credits in the series itself.