Kai Ishio is a river god of a river in the nearby mountains. Lonely in the mountains, he came down to live among humans, acting like a normal child. As a human, he took the name Kai Ishio (石尾カイ, Ishio Kai).
He is very fond of Natsume, and despite feeling he might have betrayed him for humans, looked favourably upon him.
As he left to the mountains once again Kai left a parting gift of a holy wind chime hanging in Natsume's window. It is suggested by the dialog that Taki Tooru and Takashi Natsume later go and visit him in his mountain.
Appearance
Kai is a youkai that can be seen by humans, probably due to his own powers. Kai looks like an ordinary human boy, with black-grey colored hair and eyes.
His right eye can't be seen because of his bangs that cover it. He usually wears a simple white shirt and shorts.
Personality
Unlike most youkai Kai is shown within the episode to have near human emotions. He is also known as "Crybaby god" by his worshipers for the frequent floods that spill from the rivers.
Obviously prone to outbursts and driven to loneliness in his mountain home, Kai is one of the more affectionate youkai shown in the series.
History
Apart from that the general consensus of information is that he has inhabited the mountain since his creation.