Rongo, a water spirit and daughter of earth deity Ahem, was once betrothed to Ongo, the spirit of wood and destruction, until his malevolent essence fractured their bond. Centuries later, detecting Ongo’s resurgence in Tokyo alongside Mii—a flower spirit occupying the body of teenager Natsumi—Rongo, consumed by envy, possessed Natsumi’s friend Nami to sabotage the protective skull necklace shielding her rival. Thwarted by an electric surge, Rongo retaliated by channeling her power through Nami’s hypnotic dance, summoning torrential assaults to challenge Natsumi. Her onslaught nearly destroyed a fighter jet before Ongo intervened, his interference accidentally shrinking her into a powerless child form. This metamorphosis rekindled Ongo’s dormant memories, compelling Rongo to seek refuge with Natsumi in a tentative truce.

During Ongo’s subsequent descent into chaos, Rongo’s faltering resistance gave way to Mii’s decisive confrontation. Her mastery over water, expressed through fluid, dance-like transformations and explosive hydrokinetic bursts, remained her defining trait. Supplemental narratives, including drama CDs, reveal her clandestine efforts to reclaim Ongo’s attention—infiltrating a space shuttle alongside him and scheming with gifts to eclipse Natsumi—blending rivalry with fragile collaboration.

Rooted in myth, Rongo’s lore echoes Momoke, an Oceanic water deity, while her name evokes Rongo-mā-Tāne, a Māori god linked to cultivation and conflict. Her design, marked by cascading blue hair and aquatic motifs, mirrors her elemental essence.