If a story has both mer-society and underworld hierarchies, the protagonist is almost always torn between two separate sets of rules and obligations. The sea side might demand loyalty to a royal court or a specific territory, while the underworld could involve pacts, debts, or a hierarchy of infernal power. I'm thinking of stories where the mermaid's siren nature is actually a demonic trait, and the central conflict becomes about which heritage defines them.
For example, a narrative might have them broker a ceasefire between warring ocean clans and underworld factions, using their unique position to translate motives. The physical environment also creates tension—negotiating in a sunken temple versus a sulfurous cavern demands different skills. The resolution often hinges on them forging a third path, neither fully sea nor hell, but something new that forces both worlds to adapt.
It's less about choosing a side and more about the exhaustion of constantly code-switching for survival.