A Sennin (仙人) is a Doushi that has already reached enlightenment and no longer needs to train further in their art. After becoming a Sennin, they are allowed to take on disciples.
The chances of Sennin conceiving offspring with each other has a 0.000000003% success rate. The only known Pure-Blooded (natural-born, and not trained) Sennin, is Ryuukitsu Koushu. In such cases of a Pure-Blooded Sennin, just being in the human world puts a great strain on them, as the air there is poisonous to them. They need the pure air of the Sennin World.