Poseidon is one of the Ten Elders and the reincarnation of the Greek god. He can float and create high-pressurized blasts of water from his hands, and wearing only a loincloth earns him the nickname "Nudist" from Princess Suya. After he was reincarnated due to humans straying from worshiping the gods, Poseidon had to start over with a childlike appearance and was taken in by the Former Demon King Oxenthree, who raised him alongside his own son (Twilight), and Poseidon's older brother who also reincarnated, Hades.

Even though Poseidon is less likely to tolerate Suya's eccentricities, he still falls for her tricks and schemes. Once a year, during Marine Day, the increase in magical energy allows him to take on a more mature form (which Suya doesn't recognize him in), though his mind remains the same.

When Zeus visits the Demon King Castle and decides to stay, Poseidon is elated at the opportunity to act like an older brother. However, since he's never done it before, Poseidon gets easily flustered whenever he's around Zeus and doesn't know how to express his affection.