Susugi is a youkai whose name was stuck to Tsuyukami’s name in the Yuujinchou.
In the past, he travelled from village to village through the shadows and, in exchange for food, he washed dishes for people.
One day, he met Reiko Natsume who challenged him to a fight. Her proposition was that if he won, she would buy him some food. Of course, like the other youkai before him, he lost and signed his name.
However, due to the modern facilities, even nights nowadays are as bright as daylight. Because of that, he decided to move back to the mountain.
Nyanko-sensei mentions that he have many friends on the mountain, therefore Natsume need not worry about him.