Poseidon reigns as the Greek God of the Seas and a dominant Olympian. Bearing titles like "Most Fearsome God," "God of Gods," "Zeus of the Seas," and "Tyrant of the Ocean," he commands absolute dominion over the oceans. His nature embodies extreme arrogance and emotional detachment, coupled with an unwavering belief in divine perfection. He declares gods inherently flawless, needing no alliances, schemes, or support. This philosophy fuels his contempt for humanity, viewed as filth, and his scorn for other deities, dismissed as "useless bottom feeders" or "herd."

Third son of Kronos and brother to Zeus, Hades, and Adamas, Poseidon displayed profound apathy towards Adamas throughout their existence, consistently ignoring his desperate pleas for acknowledgment. When Adamas attempted a coup against Zeus, Poseidon intervened solely to uphold his ideal of godly perfection, mortally wounding Adamas with a single strike. He then erased Adamas from official records, reducing the Olympian council from thirteen to twelve. However, motivated by concern for Hades's feelings rather than mercy, Poseidon permitted Hermes and Hades to retrieve Adamas's body, allowing Beelzebub to heal him and grant a new identity in Helheim.

Poseidon's relationship with Hades starkly contrasts his treatment of Adamas. He holds genuine respect and displays rare emotional vulnerability towards his eldest brother. Witnessing Hades single-handedly defeat the Titans in Helheim, Poseidon expressed uncharacteristic concern for his safety beforehand and afterward smiled while addressing him as "Brother" for the first time. This bond directly influenced his decision to spare Adamas, preventing Hades's inevitable grief.

Representing the gods in the Third Round of Ragnarok, Poseidon faced the human swordsman Kojiro Sasaki with utter disdain, initially refusing to acknowledge him. He fought with blinding speed and precision using his divine trident, materialized at will. His techniques, named after Greek figures linked to him, included "Amphitrite" (a rapid circular barrage of thrusts), "Divine Lightning Falling into the Rough Sea" or "Chione Tyro Demeter" (aerial strikes resembling a thunderstorm), and "Forty Days and Forty Nights of Flood" or "Medusa Alope Demeter" (a dome of afterimages attacking from all directions). Despite overwhelming Kojiro early on and shattering his sword, Poseidon's underestimation proved fatal. Kojiro adapted, predicted his movements, severed both his arms, and ultimately bisected him vertically. Poseidon perished cursing Kojiro with his final breath, becoming the first god defeated in Ragnarok.

His divine physiology granted semi-immortality, physical prowess surpassing humans, and immunity to mortal weapons. He possessed phenomenal strength, effortlessly impaling gods and breaking divine weapons, and unfathomable speed, creating afterimages with his trident techniques. He demonstrated hydrokinesis by parting seas during his arena entrance but never utilized water manipulation offensively. His endurance allowed him to fight calmly despite severe injuries, continuing his assault even after losing his arms.

Following his death, the gods showed little mourning. Zeus acknowledged Kojiro's superior strength, while Hades later sought vengeance for his brother's defeat. Poseidon's legacy endured through his devoted servant Proteus, who revered him deeply and implored Hades for retribution.