Isozuki no Nezumi is a polite youkai that can turn into rat. His master ordered him to bring the egg of the Tatsumi youkai, the one that Natsume took home. The reason is that meat of young Tatsumi youkai is very delicious and it's blood can give eternal youth.
The owner of the inn that Natsumi Takashi goes to in episode 8. It is told that while she was young, she came to like a person, known as Keiichi, who was around Takashi's age. However, Keiichi was sick. One day when he fell seriously ill, Chizu went to Sasafune to get mermaid blood. She fed Keiichi the blood and he was later on transferred to a faraway hospital. Thinking that she had made Keiichi's life miserable, she spends everyday waiting for him at a train station. It is revealed at the end of the episode that Sasafune did not give a vial of mermaid blood to Chizu. It was a vial of grape juice.
A spirit that resides in the abandoned school building, where Natsume and his schoolmates gathered to hold a last test of courage because the school was about to get demolished. Shigure used to be a young god of fortune, who was captured and imprisoned by a greedy merchant in the past.
Hishigaki and Karikage were two youkais that been chasing Natsume until he found shrine where Madara was sealed. After discovering the Book of Freinds he reutned Karikage's name, however in the anime Karikage was defeated and chased away by Madara.
A youkai whom Natsume meets in the first episode.
Appears in episode 2. She is a Youkai banished from the village of Ukihara