Ruka is a white oceanic dolphin. She becomes instrumental in ensuring the survival of Triton, the last member of the Triton clan, after his family is annihilated by the Poseidon clan 5000 years ago. Rescuing the orphaned infant Triton from Atlantis, Ruka ferries him to the coastal caves of Japan for safety. She remains Triton's lifelong companion and guardian.

Accompanied by her own offspring, Uru, Karu, and Fin, Ruka and her family form Triton's core allies in his quest for vengeance against the Poseidon family. Demonstrating protective loyalty, she actively participates in Triton's battles and strategic efforts against Poseidon's forces.

Following Triton's marriage to Pipiko, the last surviving mermaid, Ruka safeguards the seven eggs Pipiko lays, ensuring the continuation of Triton's lineage. She remains present during the events involving Triton's children.