Man, that's a fascinating angle. I think a lot of people focus on the romance when they see 'mermaid prince,' but the worldbuilding potential is wild. His identity as a royal hybrid—part of two worlds but never fully belonging to either—would fundamentally color how he rules. Imagine a kingdom built on the paranoia of being discovered, or perhaps on a deep-seated resentment toward the surface. He might enforce brutal isolationist policies, turning the sea into a fortress. Or, if he's more idealistic, his reign could be defined by dangerous attempts at diplomacy, trying to bridge a gap his own people see as betrayal. The court intrigue would be next-level, with traditionalist mer-nobles constantly questioning his loyalties. I read a serial once where the prince's half-human nature made him obsessed with collecting sunken human artifacts, which he used as both a cultural study and propaganda tools to alternately demonize and romanticize the surface. It wasn't a stable kingdom; it was a deeply insecure one, reflecting its ruler's own fractured sense of self.
You'd also see it in the kingdom's aesthetics and laws. A prince who knows human love might outlaw the very siren songs that lure sailors to their doom, creating a moral schism with older, more predatory merfolk traditions. His palace wouldn't just be coral and pearls; it'd have salvaged ship figureheads, stained glass from wrecks, a library of waterlogged books—a constant, haunting reminder of the other half of his soul. That internal conflict becomes the kingdom's foreign policy and its cultural heartbeat. The waters around his territory might be strangely calm, or marked by strange magics meant to hide or to tempt, all direct extensions of his personal turmoil. It's less about him having a kingdom and more about the kingdom being a living extension of his identity crisis.