Actually...
[spoilers] Some of the characters use a very complex technique to ensure there will be others like "them" in the future. For instance, Yukari Yakumo. She does a complicated technique of transmigration to a current generation that carries the same personality. When that "someone" arrives, she waits for her to reach maturity in terms of capacity. Once that happens, she shall fight her and if she is defeated, she will pass on her name, power and memories to the one who defeated her, thus retaining the existence of Yukari Yakumo. Remember, Yukari wasn't originally a youkai and probably has a diffent name before. Guess who's the next Yukari Yakumo. [/spoiler]
Nah, I'm just making things up...