Thank you so much! :D
I would say that he's a mythological creature that is sometimes associated with mermaids. I can't reveal exactly what he is now, but you're not too far off.
Yeah, MC isn't exactly very bright in this situation.
There are some playthroughs people did of the game on Youtube. They managed to get Ending 3, so if you need some guidance, you can check out their videos. I can link some of them if you like.
Thanks again for the comment and for loving the game! ^^