Poseidon apears in Kid Icarus: Defender of Light.
He is supreme ruler of all of the ocean and it's creatures. He often feels he has rights to judge others based on their actions, which evenntually leads to a war with Zeus.
He commands various sea monsters known as Poseidon's Army. He is a overall righteous being, but he dislikes the ocean being misused or poluted by Underworld Enemies or selfish humans.
He aids Pit and Dark Pit in Chapter 7, lowering the sea to allow entrance to the Stormsea Palace to defeat Precipitous.
In Chapter 15, Pit and Dark Pit seek Poseidon as an alliance against the Underworld Army. Upon a short conversation with Zeus, who recently allied with them, they begin arguing about each other's negative actions, and soon start a war: Posiedon's Army vs Elemental Army. In the end, however, both see each other as good allies and join together with Pit.
In Chapter 22, Posiedon,the only god Pit and Dark Pit trust not captured and under the Chaos Kin's influence, helps Pit and Dark Pit by giving them flight and sending them to the Galactic Seabed to retrieve the Space Pirate Ship. He sends his army to repair the ship, allowing them to fight against Hades to stop the Chaos Kin.