YC is a merman creature of some sort. Preferably a 'non traditional' fish or sea creature of some kind. Think a merman that resembles an Oarfish, Anglerfish, Coelacanth, etc [the weirder, the better]. He has been exiled from his pod because of [whatever reason, up to you] and has been living on his own. He is regularly picked on by standard mer people because he's an oddball and doesn't belong, but also has never lived outside of his previous pod before and is barely surviving; he also misses his them and ultimately wishes to do whatever he can to be accepted again, mostly out of loneliness.
MC is a merman of the octopus variety. An apex predator and highly intelligent. Dangerous but also strangely playful and curious as well as completely alien to YC [and each other]. Initially finding YC out in the open, it seems like an easy lunch, only to realize that he's never seen a mer like YC before and despite being unconventional compared to the usual mer-types where he lives, he finds YC to be curious and even cute. [To YC's horror, as MC again, is a predator]
This would start as a highly unconventional enemy-to-friendship-to-romance situation.
Edit: I want these guys to be pretty monster-y. Doesn't have to be scary or anything, but not traditional 'Ariel' or 'Ursula' merfolk. Feel free to DM and I can send over examples/my own character to clarify.