Koh the Face Stealer is one of the most ancient and knowledgeable spirits in all of the Spirit World. Nearly as old as the Ocean and Moon Spirits, he is the only known individual to remember them crossing over to the physical world, taking the form of koi fish. He is a malevolent entity residing under an ancient tree in the Spirit World. Possessing the body of a gigantic centipede-like creature, Koh can change the appearance of his face to that of any in his collection at will by "blinking", and will steal the face of anyone or anything that expresses emotion.
When Ummi and AvatarKuruk were getting married, Koh lured Ummi to the Spirit World, where he stole her face and trapped her forever as punishment for Kuruk's arrogant ways. Kuruk was determined to end Koh's life, but when the two met, the past Avatar could not go through with it as he realized that bringing an end to the spirit would also destroy the love of his life. Despite Kuruk's vindictive actions, Koh himself held no personal grudge against Aang, as he was a different incarnation of the Avatar.
During the Hundred Year War, Koh stole the face of a young Northern Water Tribe citizen, Rafa, though Koh's mother later gave the young man a new face.
Aang appealed to Koh for information regarding the Moon and Ocean Spirits, Tui and La, where Koh warned the Avatar of impending danger the spirits faced. When Aang asked for their location in the mortal world, the old spirit did not provide a clear answer and merely stated that they were opposites, forever revolving. When he told Aang that he had already met the spirits, the Face Stealer barely missed Aang's expression of epiphany when he realized that the identities of Tui and La were the koi fish in the Spirit Oasis.
Before Aang left, Koh stated they would meet again, which they did when Aang found himself in the Spirit World after being struck by Azula's lightning in the Crystal Catacombs of Old Ba Sing Se. During that encounter, Koh attempted to steal the young Avatar's face before he reconciled with the other past Avatars, but failed.
As an ancient spirit, Koh is a perilous force not to be trifled with. Koh steals the face of anyone or anything that shows emotion to him, which is why those who approach him are warned to remain inexpressive while in his presence. He possesses a sadistic personality, constantly trying to goad the slightest bit of emotion out of his visitors by frightening them with his monster-like appearance, or taunting them with his many faces.
However, Koh's actions suggest he was amoral rather than purely evil. He stole Ummi's face to punish Avatar Kuruk for his arrogance, but willingly told Aang the location of the Moon and Ocean Spirits and the danger they were in, and led him to his past Avatar lives. He is cryptic and often speaks in riddles.
These are some of the millions of faces stolen by Koh. They are listed as they appear in the picture, from left to right.
A female Noh mask, a white face with red lips and gray eye markings. This was the face that Koh wore most often.
Koh stated that he stole Ummi's face "nearly eight or nine hundred years ago", when in actuality Avatar Kuruk lived only about four hundred years before.
Even after Koh steals an individual's or animal's face, the creature still survives, as seen with a monkey who was still outside Koh's lair despite losing its face. This also holds true for beings in the human world (such as Rafa), though they will enter a state "somewhere between life and death" that can persist for years.
His name resembles the Japanese word "Kao" (顔), which means "face".
Koh's abilities are diametric to his mother's; he has the ability to steal faces, whereas she has the ability to give faces.
Legends say that while on strained terms, Koh steals faces as a way to be close to his mother.
Gene Yang, who considers Koh to be "one of the best villains ever created," wanted to find a way to include him in The Search due to the trilogy's prevalent theme of faces, thus making him happy to connect Koh with the Mother of Faces.