So we have the following options:
1. Most people guessed right and it really is Megumi
2. Harem end
3. Namori is trolling us and we'll never find out who the mystery kanojo is
I suggest that if No. 3 comes true we riot and go jellyfish hunting, who's with me?
If the same author were to create another doujin's summary like: Koishi went somewhere else and never return, the hole inside Frandre's heart is never filled ever since. Something like this. I think my room will be filled with my tears.