Demons in general are immortal souls.

So what are Youkai demons?

They are souls of people who died twice, given another chance by Nemesis and Hestia to roam the world again, thirsty for revenge or to soothe their good but troubled souls.

Youkai Demons are divided into the ones driven by revenge, Hunger, and the ones driven by their desire to help change the world/make amends with someome, Satisfaction. Hunger Yodes are malicious and can only be defeated by convincing them to stop. They can then pass on, leaving no Yode medal. On the other hand, Satisfaction Yodes try their best to help others, but are very determined when facing adversaries and comforting their partners, making them good partners.

Usually, Yodes are recycled from Youkai souls who died, and it shows with their need for a medal and a watch.

Matsukaze Tenma, Tsurugu Kyousuke, Shindou Takuto and Kirino Ranmaru are all acknowledgers of this species.