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Kosokoso appears as a young girl, with dark shadows under her eyes, fangs in her mouth, fair skin, and long dark hair. She also has a third eye that is normally hidden on her forehead. She wears a kimono with a five-petal flower patterned over it, a dark-colored obi around her waist, and a pair of geta sandals with tabi socks.
At some point in the past, Kosokoso learned about the secret of the Ancient Demons of the cave she was guarding. She made a deal with them where after she awakens them she will conquer the world with their powers. She aimed to get the "Blue Blood" that will awaken them, and late found out that Sayuri Kimura is the current vessel of the blood.
She was the actual cause of "Kosokoso-Iwa" to make "sneaky" voices midnight, and this was to scare off humans and to secure the cave which was connected to underground areas where the ancient demons were hiding and sleeping, and the boulder was guarding the cave entrance. She kidnapped Sayuri, and Kitarō knew this by insects' informing and came to rescue Sayuri. Kosokoso was at the upper hand with her petrification beam, and Kitarō's hands and legs were turned into stones. Kitarō's friends arrived to aid Kitarō, but they all were petrified.
She was nearly killed with Kitarō's stomach acid and half of her body was dissolved, but she stood at her last resort and sucked Sayuri's blood, and spewed on the demons before her death, and she succeeded to revive the demons at the cost of her own life.
Powers and Abilities
Kosokoso Beam (こそこそ光線 Kosokoso Kōsen): Kosoko fires a beam from her eyes that can petrify whatever is caught in it. She is capable of using this with her third eye if her main eyes are blinded.
Sleep Inducement: She can make her targets fall asleep by tickling them.
Bloodsucking (吸血 Kyūketsu): Kosokoso is able to suck the blood from a human with her fangs and is also capable of spewing it out.
- Her design is based on the girl in Kosokoso-Iwa's illustration.