Poseidon is shown to be a neutral party, not wishing to get involved in the affairs of others. However, as the proud God of the Sea, he refuses to tolerate others trespassing on his domain, and gladly lends his support to Palutena to help her rid the ocean of Underworld forces.
"I wouldn't allow anyone else to part my ocean! In fact, I've half a mind to put it back together!"
https://kidicarus.fandom.com/wiki/Poseidon