Koh the Face Stealer was one of the most ancient and knowledgeable spirits in all of the Spirit World. Nearly as old as the Ocean and Moon Spirits, he was the only known individual to remember them crossing over to the physical world, taking the form of koi fish. He was a malevolent entity residing under an ancient tree in the Spirit World. Possessing the body of a gigantic centipede-like creature, Koh could change the appearance of his face to that of any in his collection at will by "blinking". Upon seeing anybody or anything express emotion, Koh would steal its face.
As one of the oldest spirits, Koh possessed a vast amount of knowledge of the spiritual realm and the beings which resided within it. 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.
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.
As an ancient spirit, Koh was a perilous force not to be trifled with, though the extent of his power was never shown. Koh stole the face of anyone or anything which showed emotion to him, which was why those who approached him were warned to remain inexpressive while in his presence. He possessed 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 suggested 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 was cryptic and often spoke in riddles.
Koh's bad breath almost made Aang crack while not showing emotion, despite the former's frightening faces.
Koh stated that he stole Ummi's face "about 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. However, this occurred in the Spirit World where food, water, and air may not be necessities like in the real world.
His name resembles the Japanese word "Kao" (顔), which means "face".
Koh has several similarities to Níðhöggr, a dragon of Norse mythology which gnaws on one of the three roots of Yggdrasill, the World Tree. It is sometimes believed that the roots are trapping the beast from the world, much like the tree in which Koh lives.
He also bears similarities to the Great Old Ones from the works of H.P. Lovecraft, in terms of his ancient knowledge, demonic form, and nightmarish lair.