Sennin (仙人, Sage) is an immortal being in the Twelve Kingdoms. When a person becomes a Sennin, he or she will not physically age, preventing them from dying of old age or disease. However, if the person loses the sennin status, he or she will age normally.
In the Twelve Kingdoms, sennin are individuals who are granted divine status. Rulers become sennin when they ascend the throne, and most make their advisors sennin as well. Once a person becomes sennin, their name is entered into a kingdom's Divine Registry (神籍, shinseki); removing an individual from the Divine Registry will immediately revoke the person's sennin status.
Sennin cannot die by ordinary means as a result of their divine status. It is known that touki weapons, or other extraordinary weapons such as Youko's Suiguutou, possess the ability to kill sennin. Sennin can also be killed if they are fatally injured by youma or if they are beheaded or cut in half. A sennin is also granted the power to understand different languages, which only seems relevant when dealing with kaikyaku and sankyaku, animals, or people with impaired speech.
Ordinary people can become sennin by being granted the status by a sufficient ranked sennin, such as the ruler of their country. Other ordinary people are able to achieve sagehood on their own power and ability.
- Heaven's Sage (天仙, tensen) - Holy immortals who serve directly under the Heavens. They include the monarchs and kirin of each kingdom and possess the ability of all sennin, though they can only die if they are beheaded or have lost the way. They have the power to cross over to Hourai via a shoku.
- Oracles (女仙, nyosen) - Female sages who care for the kirin when they reside on Mount Hou. They achieve their status by fasting and praying to Seioubo.
- Flying Sage (飛仙, hisen) - Sennin who are do not serve a kingdom's monarch. They may live as they see fit; while some elect to serve the gods, others simply live in seclusion from the rest of the world. While some flying sages obtain their status from a monarch elevating them, others achieve immortality through their own ability. Flying sages who acquire their status on their own are automatically granted the rank of "count" and are powerful enough to cross the Void Sea. Sennin who have ascended by the edict of a monarch like Lady Riyou of Sai or nyosen will only have the rank of viscount.
- Grounded Sage (地仙, chisen) - Sennin who participate in the governance of a kingdom. Most are granted their status by the monarch of a kingdom.