Tennou is the title given to the absolute ruler of Oniga; its king, in a sense. They possess absolute rule over the people; their word is like that of a God, and the people respect the royal family as if they were actually gods.
When it comes time to pass on the title, due to death or the failings of age, it almost always falls on to the shoulders of the eldest son of the royal family. If there is no such individual, which is beyond rare, it falls to the Tennou's brother, with priority given to age if there are multiple brothers.
If that were to also somehow be impossible, in that very rare circumstance, it would be the responsibility of the eldest daughter of the Tennou to become the new Tennou. Realistically, a female Tennou would be an outrage to ruling courts, and it would be a messy affair, but if the individual proved themselves capable, those courts would have no recourse. If all of those should fail, then one of the heads of the ruling houses would have some method of deciding who would be the new Tennou.