Poseidon rules the seas as the leader of his clan and serves as the primary antagonist. His envy drives him to systematically annihilate the rival Triton clan in Atlantis, leaving Triton as the sole survivor; this genocide stems from resentment over the Triton family's successes and peaceful existence.

To achieve his goals, Poseidon orchestrates a devastating tsunami that destroys a coastal Japanese village, claiming the life of a human caretaker's father and demonstrating his willingness to inflict collateral damage on humans. His cruelty extends to manipulating human affairs, enabling circumstances that force Kazuya into becoming a fugitive unknowingly working for Poseidon's supply operations.

Commanding numerous offspring and monstrous minions, Poseidon repeatedly deploys them to eliminate Triton. The death of his 33 children during Triton's quest for vengeance escalates Poseidon's personal vendetta. Faced with failure, he resorts to deception out of desperation, creating a destructive new son using a sponge disguised as Triton's ally, Pipiko. This monstrous offspring spirals out of control, wreaking havoc across the oceans.

The uncontrollable creature forces Poseidon into a pragmatic truce with Triton, negotiated under the witness of the ancient turtle Ganomosu. The agreement stipulates Triton will eliminate the monster in exchange for Poseidon ceasing hostilities against merfolk. Despite this temporary ceasefire, Poseidon later engineers a scheme to frame Triton for catastrophic tsunamis attacking Tokyo, manipulating media coverage and exploiting Triton's emotional distress over his missing child.

His final confrontation unfolds within his underwater base, where he battles both Triton and Ganomosu. The conflict culminates in a massive explosion that destroys the base, resulting in Poseidon's demise and Triton's death, fulfilling his role as an enduring symbol of tyranny and vengeance.