Poseidon is the God of the Sea, debuting in Kid Icarus: Uprising.
While he remains unaffiliated, he serves as a temporary ally to Pit during his brief appearance in Chapter 7.
Kid Icarus: Uprising - Idol Description
"The god of the sea. A silent observer of the invasions of the surface world, Poseidon grows annoyed with the Underworld's antics and grants a request to aid Palutena by opening the way to Thanatos's Seafloor Palace."