Natsume's Book of Friends

Natsume encounters a young boy named Kai sealed in a coffin in an abandoned house. Natsume sets Kai free and finds out that Kai is being stalked by a youkai. After a time, Natsume becomes the target of this youkai and stops the youkai's stalking with Nyanko-sensei's help. In the end Natori who turns out to be the stalker and who sealed Kai in the coffin, tells Natsume that Kai is a Youkai. (Source: Wikipedia)