Wan Shi Tong

 "spirit"

- I am Wan Shi Tong, he who knows ten thousand things. And you are obviously humans. Which, by the way, are no longer permitted in my study.

Wan Shi Tong, Knowledge, is an ancient, wise, and powerful being. He collects information for his Spirit Library, and thus is more knowledgeable than any human being. Although his regular shape is that of an immense black-and-white barn owl, when angered he can assume much more frightening forms.

History
Long ago, Wan Shi Tong brought his Library from the spirit world to the physical world. Together with his band of knowledge seeking foxes they collected knowledge from all over the world. He put it all on display in his library for humanity to read and hopefully better itself.

Although he left his great collection of knowledge available to the mortal world for many years, he is recently rather angry at humankind; he apparently sees knowledge as something good purely for its own sake, and in recent years he is known to have been visited by humans only twice — once by Zhao (who was then a young lieutenant) and once by Aang and his friends — in both cases because the visitors were hoping to find secrets to destroying their enemies (the Water Tribes, and Fire Nation, respectively). After Sokka breaks a vow made to Wan Shi Tong that no knowledge found in the library will be used for violence, Wan Shi Tong pulls the library back into the Spirit World by sinking the library, attacking Aang and the others. He was eventually knocked out by Sokka who then, along with Aang and Katara, escaped from the sinking library.

Trivia

 * "Wan Shi Tong" (萬事通) is a phrase in Chinese that means "he who knows 10,000 things", which is what he had called himself in "The Library".
 * The "10,000 things" is a Taoistic expression for all of creation, introducing himself as "he who knows 10,000 things" Wan Shi Tong calls himself all-knowing. He also directly refers to himself as "all-knowing" when he tells Sokka "If you're going to lie to an all-knowing spirit being, you should at least put some effort into it."
 * Wan Shi Tong's two forms are both based on animals used as symbols for knowledge in the real world, the owl (west, such as Europe) and the serpent (east, such as China).
 * Like an owl, Wan Shi Tong has the ability to turn his head almost completely around.
 * When Wan Shi Tong first appears in "The Library", he briefly looks like the spirit "No-Face" from Miyazaki's "Spirited Away". Both No-Face and Wan Shi Tong turn into a more dangerous and evil form when near one of the Seven Deadly Sins (Wan Shi Tong turned into a dragon-like beast when he saw what Aang did as Wrath or violence and No-Face turns into a grudge monster form when he accidentally absorbs the employees greed).
 * Wan Shi Tong is voiced by Hector Elizondo, who plays Joseph in The Princess Diaries and Bane in Batman: Mystery of the Batwoman.
 * Wan Shi Tong only appears in the episode "The Library" but is mentioned in a few other episodes.
 * Wan Shi Tong also appears briefly in the animated short "School Time Shipping", as the school librarian, parodying his library in the regular series.
 * Wan Shi Tong represents himself as an Owl, in possible reference to Athena the Greek Goddess of Wisdom's symbol, the Owl.
 * Wan Shi Tong greatly resembles a common barn owl, which are known for their excellent hearing and flat facial disks.
 * Wan Shi Tong has studied all three types of Waterbending: Northern Water Style, Southern Water Style, and Foggy Swamp Style.
 * Owl is a form of wisdom in Asia.
 * Wan Shi Tong's normal form as a barn owl is itself a reference to his spirit status; Barn Owls, because of their pure white faces and black eyes, are often called "Ghost", "Demon", or "Death Owls".