Ugayafukiaezu was a child of Hoori, the son of Ninigi-no-Mikoto, who was sent down by Amaterasu to govern the earth (Ashihara no Nakatsukuni) (believed to be equivalent to Japan), and of Toyotama-hime, a daughter of Ryūjin, the dragon kami of the sea.

[2] Although Toyotama-hime became pregnant at the undersea palace of Ryūgū-jō, she opted not to bear the child in the ocean and decided to head to shore.

"Now, thinking these words strange, he sneaked up and peered in at her just as she was about to give birth.She had become an enormous sea beast many arm spans in length that was twisting and slithering around on its stomach.In shock and fright at the sight of her, he immediately fled far away.

Later, when Ugayafukiaezu reached adulthood, he married his aunt, Tamayori-hime, and they had four children: Hikoitsuse, Inai, Mikeirinu, and Hikohohodemi (later Emperor Jimmu).

[1] Ugayafukiaezu is in the Three generations of Hyuga, a time period between Tenson kōrin and Jimmu's Eastern Expedition.