Leto is a demon in the series.
In Greek Mythology, Leto was one of the Titanides (female Titans), and the daughter of the Titans Coeus and Phoebe born on the island of Kos. Her name and iconography suggest she was also a goddess of modesty and womanly demure. Like her sister Asteria she may also have been a goddess of the night, or alternatively of the light of day.
She was one of the brides of Zeus, and the mother of the twin gods Apollo and Artemis. When Leto was pregnant with the twins she was pursued relentlessly by the goddess Hera, who drove her from land to land preventing her from finding a place to rest and give birth. The floating island of Delos eventually provided her with refuge. Later when she was later travelling to Delphi, the Gigantes Tityus attempted to abduct her, but Apollo intervened and slew him with arrows.
In Greek vase painting Leto was usually depicted as a woman lifting her veil in a gesture of modesty. The exact meaning of her name is obscure, some commentators connect it with the word lethô, to move unseen, suggestive of modesty, others derive it from the Lycian word for woman, lada.
Kyuuyaku Megami Tensei
Leto is one of four members of the Deity race that are exclusive to the updated version of Megami Tensei II, alongside Zeus, Apollo, and Artemis. Her stats and skillset are near identical to Baal's, the only difference being slightly more HP. The hidden deities can only be found on the "Bad ending" route, which requires killing Bael as a frog. Leto is found the area of Hades that the portal in Shinjuku's Underground connects to.
Kyūyaku Megami Tensei
Megami Tensei II