Mortals are so predictable...and such terrible liars.
~ Wan Shi Tong after discovering Sokka betrayed his trust
You betrayed my trust. From the beginning you intended of misuse this knowledge for evil purposes. (Sokka: You don't understand! If anyone's evil is the Fire Nation! You saw what they did to your library, they're destructive and dangerous! We need this information!) You think you're the first person to believe that their war was justified? Countless others before you have come here, seeking weapons or weaknesses or battle strategies.
~ Wan Shi Tong, about that the Hundred Year War dosen't justify for Team Avatar to use his library to defeat the Fire Nation.
Wan Shi Tong, which is translated as He Who Knows 10,000 Things, is the Spirit of Knowledge in the world of Avatar the Last Airbender. Despite his noble position, he is cynical, xenophobic, and hostile towards the Avatar and their allies, thus making him a villain, despite his honest desire to preserve the knowledge he has accumulated.
Avatar the Last Airbender
Wan Shi Tong first appears in the second season Avatar the Last Airbender in the episode "The Library". He oversees perhaps the greatest library in the world, but does not allow humans to enter based on the cynical belief that all humans who seek knowledge simply do so to gain some kind of advantage over an enemy. Though Aang and his friends are initially able to convince him otherwise, Sokka goes out of his way to locate a means to stop the Fire Nation, which enrages Wan Shi Tong. Despite Aang's and his friend's pleas that the Fire Nation must be stopped, Wan Shi Tong tells them that countless humans before the Hundred Year War came to his library, seeking knowledge or weapons to defeat their enemies, while believing that their wars were "justified". He decides to finish burying his library under the desert sands "so that no one will abuse his knowledge again". He also attempts to kill Team Avatar on the grounds that they "know too much". Fortunately though, Aang and his friends manages to escape in time, but Professor Zei, who had spent his entire life looking for the library, chooses to stay.
Legend of Korra
Wan Shi Tong appears later in The Legend of Korra. His library now located in the Spirit World. He is leery of a visiting Jinora when she shows up asking for information on the Spirit Portals and swiftly sells her out to Unalaq after finding out she's both the granddaughter of Avatar Aang, and friends with the next Avatar Korra, implying that he still bears a grudge against the Avatar for abusing the knowledge in his library. He has also come to believe that Unalaq is more of a "true" ally to the Spirits than the Avatar.
Wan Shi Tong is known as the Spirit of Knowledge, making him debatably one of the most intelligent beings of all times. And being a spirit makes him immortal. He can fly, and has razor sharp talons and beak. He also has some signs of shape-shifting, as seen when he's attacking Aang and his friends. His neck is elongated, gains feathers shaped like horns, and his body becomes longer and more sleek, making it easier for him to maneuver in tight corners.
Wan Shi Tong bears a strong resembles towards No-Face from the Miyazaki film Spirited Away.
The reason why Wan Shi Tong hates humans from entering his library is most likely explained in the Episode; The Siege of the North, Part 1. Years ago, Wan Shi Tong let Zhao in his library, resulting in him learning about the location water and moon spirits; Tui and La and burning part of his library. At the very least, he uses this as an example to illustrate his point.
Wan Shi Tong is voiced by veteran actor Hector Elizondo.