Koh the Face Stealer is a centipede-like spirit creature with cockroach-like legs, and an eye-shaped head with eight spider leg eyelashes/arms. Yeah. Koh is really, really creepy. Adding to this, Koh has the ability to steal anyone's face -- person, creature, or otherwise -- and use it as his own. The stolen faces will appear in the center of his head (or, where the pupil of his eye would be), and can change the face to another at will by "blinking" his head's center eyefold. The only way to speak with Koh and not have your face stolen is to show no emotion at all.
Koh also seems to be cunning, heartless (hello, he steals faces!), and very knowledgeable. He is also very, very old, possibly even dating back to "near the beginning", as Avatar Roku said Koh was the only spirit he knew of that would remember the Moon and Ocean spirits' mortal forms.
Koh lives in the swamp area of the Spirit World, underneath the base of a giant gnarled tree. It's interesting how the sky above the tree has the brightest light in the entire swamp, while Koh lives in the darkest depths in the roots.
As Avatar fan Sharon pointed out to me, this may have been influenced by the Norse legend of the World Tree Yggdrasil. In (part of) the legend, a wise golden eagle lives in the treetop while in the roots lives Nidhogg, a giant serpent/worm/dragon creature that gnaws on the roots and has a taste for people. o_0
Koh has had only one appearance in the show so far, in episode 20. But I have a strong feeling that Koh will return again in the future. Though, that probably won't be a good thing for Aang or anyone else. ^^;
Koh the Face Stealer
Koh's Face Collection
(That we have seen so far)
This is the first face Koh is seen wearing, and he changes back to it again later on during his chat with Aang.
This mask looks very similar to the Japanese Noh masks; especially the one called "Magojiro". Except, Koh's mask has large gray circles around the eyes, where as the Noh masks in this style do not.
This is the second mask Koh wore; an angry looking older man. Judging from the style of his beard and moustache, it would be my guess that this man used to be from the Fire Nation. Although he could be from the Earth Kingdom or Water Tribe instead.
The third mask Koh wears, and the reason why one of the previous Avatars tried to kill him. Eight or nine hundred years ago, Koh stole the face of this lady, who was loved by the then current Avatar. Clearly Koh won the fight, but it's not known what became of the Avatar.
Curly-Tailed Blue Nose
...After drawing in Aang's attention with the beautiful lady face, Koh quickly changes into this monkey's face and laughs harshly. What more, the monkey Koh stole this face from was sitting outside his lair when Aang arrived. Also, this face is very similar to that of a Mandrill monkey. Not to mention super creepy when up close.
Koh switches to this mask twice, and both times are used for when Koh suddenly zooms toward Aang and stops a few inches away from his face. Perhaps Koh was hoping to startle Aang with the mask's unusual and frightening appearance, but it didn't work.
This mask almost sort of looks like the Japanese Noh mask called "Fudo".
Koh also has the face from an owl. Which I find kind of strange, since Owls don't exactly have a range of facial emotions. Maybe it just gave Koh a funny look or something. ^^;
Koh also has the face of an old man, which he used when he was talking about how it was actually the Moon and Ocean spirits that needed Aang's help, and not the other way around. A knowledge-weary face to match a complicated problem?
Sad Young Man
The last face Koh wears as Aang leaves, and it even gets a little camera zoom before he fades into the shadows. Clearly, this face has some importance. With the similar short sideburns and beard, along with his nose and eyes, many fans believe that this face use to belong to Iroh's son Lu Ten, who had died some years back.