Originally my idea is that merfolks are people who are considered “sea children/witches” who die in the sea, and sea witches/children’s are descendants of a water spirit. The lineage is considered cursed cause when they die they supposedly turn into monsters (merfolks). But there are conditions for them to “turn”, one they perish in the water (drown) and they’re magical affinity needs to be strong enough. How they do know if a sea child is strong enough? They don’t. Sea children are scarce. Most don’t know they’re one. The Gojo clan/kamo clan/zenin clan exist as clans of sea children. But they’re recluse/scattered in the wind. So there’s a facility that’s dedicated to rehabilitating and studying Mers cause there’s just a lot of unknowns