Zeus

Zeus is the Patriarch of Olympus.
Husband of Hera.
Father of Athena, Apollo, Dionysus, Ares, Aphrodite and many others.
First appeared in Season 2.
A god terribly influenced by his own pride and his fear of obsolescence, Zeus was an antagonist for much of Season 2, becoming something more like a neutral party later in the season.
Zeus tried to entreat with Erebus, agreeing after the murder of Apollo to place Death of The Four Horsemen on Apollo's throne.
This eventually led to Death gaining greater and greater power and kidnapping Aether, the oxygen of Olympus.