Probably the MC? It's been established that the MC knew at least one of the girls as a kid, and the poster said that was 5 years ago so it lines up. Makes sense how they don't recognize him, though him taking what (9 chapters?) till he remembers the girl he met as a kid is typical forgotten childhood friend amnesia.
Yeah, God Bless Sakura Ayane. I wouldn't have started this without her, as it looks like your typical harem rom com. I mean, it still is, but I'm a big sucker for the mystery promised girl as a kid, so I'm totally in it.