Luka, a white dolphin, plays a critical role in safeguarding Triton. After the Poseidon family's genocide of the Triton clan, Luka rescues the orphaned infant Triton and transports him to Japanese coastal caves for refuge, becoming Triton's primary guardian and ally in his early years. Throughout Triton's vengeance quest against the Poseidon family, Luka remains a steadfast companion. Alongside dolphins Uru, Karu, and Fin, Luka actively supports Triton's one-man war, participating in battles and providing tactical assistance against Poseidon's forces—including assaults on their marine base and confrontations with Poseidon's offspring.

Luka contributes to key strategic moments, such as guiding Triton to the floating island sanctuary of the giant turtle Ganomosu, which serves as a pivotal refuge. Following Triton's death in the final battle against Poseidon, Luka's role shifts to the aftermath: dolphins under Luka's association inadvertently lead human mermaid hunters to the mermaid family's floating island, resulting in its destruction during a collision with the hunters' vessel.