Gōzaburō Seto commands the Seto Group, a mermaid yakuza syndicate, as its patriarch and fiercely guards his daughter Sun Seto. His obsessive protectiveness stems from rigid traditionalism, viewing marriage as a permanent transfer of a daughter’s loyalty to her husband’s family—a prospect he violently resists. This fuels his vendetta against Nagasumi Michishio, Sun’s human fiancé, whom he targets with assassination plots, sabotage, and explosive outbursts, often attacking first and dismissing evidence of the boy’s innocence.

Though prone to volcanic rage, Gōzaburō’s unpredictability veers into absurdity: he crossdresses as a schoolgirl to empathize with Sun or wreaks havoc while intoxicated, showcasing erratic self-indulgence. Paradoxically, he demonstrates loyalty to his subordinates, shielding the amnesiac Masa and prioritizing their well-being in danger. A childhood cat bite left him with crippling ailurophobia, a weakness adversaries exploit to curb his aggression.

Manipulated by Lord Yoshiou into endorsing Sun’s forced ascension to godhood via marriage, Gōzaburō rationalizes the scheme as altruistic, overriding her protests to enforce his ideals of her future. While the manga hints at reluctant growth—ceasing attempts on Nagasumi’s life by its end—the anime truncates this arc, omitting his tentative acceptance. His wife, Ren Seto, counterbalances his volatility through draconian discipline, though he quakes under her authority. Interactions with Sun swing between smothering devotion and authoritarian demands, frequently disregarding her disapproval. Yet raw vulnerability pierces his hardened exterior, such as tearful meltdowns over losing her, revealing conflicted depths beneath his tyrannical façade.