In Greek mythology, Zeus is the "Father of Gods and Men", the King of gods, who rules the Olympians atop Mount Olympus. He is also the god of thunder and ruler of the sky.
Cronos, his father, had sired six children with Rhea, his wife: Poseidon, Hades, Demeter, Hestia, Hera, and Zeus himself. But Cronos swallowed them all because of a prophecy claiming that he would be overthrowned by his son, as he had overthrown his own father. Rhea, seeking revenge, hid Zeus at birth and gave Cronos a rock wrapped in cloth to swallow instead.
Once he reached adulthood, Zeus set to fight his father. He made Cronos vomit the rock and his brothers and sisters, then he freed the Titans, the Hecatonchires, and the Cyclopes, which started a war known as the Titanomachy. Once victorious, Zeus casts his father and the other Titans to Tartarus, then divides the world between himself, Poseidon, and Hades. Zeus got the sky, Poseidon the sea, and Hades the Underworld.
Zeus is one of the four new Deity in Megami Tensei II, and is extremely similar stat-wise to Lucifer's recruitable form. To obtain him, Megami Tensei II must be started from a completed Megami Tensei save. After killing Bael's frog, he can be created by fusing Leto, Artemis, and Apollo, regardless of the protagonist's level.
Zeus is created through fusion by Elizabeth as the culmination of all her work. Upon summoning the persona his power proves too great for her, forcing itself into her and using her body to test the persona users, seeking for them to gain power of their own will without relying on brotherhood or being a mere lamb in the herd. Both forms of Zeus are used in the battle. After the fight he is overjoyed by the power and strength they show, inviting them to come to his realm one day where glory and battle awaits in a shining world beneath Yggdrasil.
This fight is only available in the post-game, after defeating the final boss, but before starting a new cycle. After defeating him, Margaret tells the myth of Zeus's and Cronos's relations to each other to either protagonist. Margaret also states that since Cronos is a symbol of death, Zeus is most likely a symbol of life, and that both teams are closely related to him.
After obtaining they key item allowing his creation, Warrior Zeus can be created through Special fusion, by fusing Michael, Satan, and Metatron. Zeus can then be created by fusing Warrior Zeus and Seth.