Sun Seto, alternatively romanized as San Seto, anchors the series’ narrative across all official media. Daughter of Gōzaburō Seto—a sea dragon commanding a mermaid yakuza syndicate—and Ren Seto, a revered mermaid warrior, she inherits a legacy blending oceanic nobility and draconic might. Bound by mermaid law requiring execution of humans who glimpse her true form, she enters a forced engagement with Nagasumi Michishio after rescuing him from drowning. Initially transactional, their bond deepens into genuine affection as she contends with her family’s violent resistance to their union.

Her striking appearance includes fair skin, tan waist-length hair with blunt bangs, light brown eyes, and a pronounced bust, drawing frequent admiration. In mermaid form, her pinkish-red tail mirrors the hues of her school uniform. A pivotal struggle emerges during adolescence when contact with water reverts her legs to a tail, a limitation she eventually masters through arduous training.

Guided by mermaid chivalry—invoking the wordplay between “人魚” (ningyo, mermaid) and “任侠” (ninkyō, chivalry)—she balances kindness, tradition, and occasional naivety. Though loyal to her heritage, she opposes her family’s lethal methods to eliminate Nagasumi, embodying the tension between familial duty and personal ethics.

Her abilities encompass the Song of Heroes, a vocal technique amplifying Nagasumi’s combat prowess, and expert swordsmanship honed under Masa, a Seto Group lieutenant. The manga’s climax unveils latent power surpassing her father’s draconic strength, positioning her as a potential successor to godhood should deity Terra abdicate—an inheritance from her dual lineage.

Her evolution shifts from reluctant compliance to proactive resolve, defending Nagasumi against rival factions and sparking a clan war to secure his rescue. Their relationship matures from a coerced marriage to mutual dedication, culminating in her heartfelt proposal following a second near-drowning.

Interpersonal dynamics include a rivalry with childhood friend Lunar Edomae and navigating her father’s overprotectiveness (notably his feline phobia, which she strategically exploits). Her mother, Ren, enforces discipline within the Seto Group while tacitly endorsing Sun’s romance.

Across adaptations, her journey interweaves humor and emotional gravity, showcasing resilience amid adversity. While the anime and manga diverge in resolving her divine potential and final conflict scale, both underscore her role as a compassionate bridge between merfolk and humanity.