"I'm not leaving ... I can't ... I've spent too long trying to find this place ... There's not another collection of knowledge like this on Earth ... I could spend an eternity in here."
— Professor Zei to Team Avatar.
Professor Zei was the head of the Anthropology Department at Ba Sing Se University. An ambitious cultural anthropologist, he was well-traveled and highly educated in exotic cultures. His thirst for knowledge brought him to strange, remote, and often dangerous places. Zei was an eccentric man to whom knowledge was priceless. He got excited at the prospect of learning anything about history or the world around him. A map of his travels showed he had been everywhere in the world, except the Fire Nation. Even though he had discovered many lost civilizations over the years, he longed to find the "crown jewel" of ancient discoveries: Wan Shi Tong's Library.
Zei attempted to find Wan Shi Tong's Library several times, but failed in every attempt. Though he knew the library was somewhere in the uncrossable Si Wong Desert, he never managed to find it on his own.
However, in 100 AG, he met AvatarAang and his friends at the Misty Palms Oasis while preparing another expedition into the desert. The professor quickly befriended the group, interested to meet a living Air Nomad. He asked questions about the Air Nomads, including their primary agricultural product, though the Avatar was overwhelmed by these questions. Sokka quickly turned the conversation toward maps, and though Zei was able to provide the group with a more up-to-date map, he was unable to provide them with one that included the Fire Nation. The team discovered that the professor had made several trips into the Si Wong Desert, and Zei informed them about his quest to find the missing library. Sokka became interested in assisting the professor in finding the library, believing that there might be information about the Fire Nation there, but Zei warned them of the difficulty in crossing the desert, noting that he had almost died in every attempt. The group decided to search the desert from the sky by riding on Appa, whom Zei was thrilled to meet, wishing that he could speak the language of the flying bison.
After a few hours of searching, they stopped to investigate what appeared to be a tall, slender tower, but upon further investigation and the observation of a Knowledge Seeker entering the structure, the tower was revealed to be the top of the library, which had been mostly buried in sand. Zei entered the library, marveling at its architecture, along with Aang, Sokka, and Katara. He easily passed Wan Shi Tong's test of knowledge by contributing a book to the library, which was found pleasing to the spirit being. The professor proceeded to peruse the library along with the rest of group, and later pointed out the Knowledge Seeker trying to help Sokka find information about the "The darkest day in Fire Nation history". Zei was greatly impressed by the mechanical marvel of the planetarium, which Sokka was using to find information that could be to the group's advantage against the Fire Nation.
When Wan Shi Tong discovered the group had betrayed his trust and were planning to use his information against the Fire Nation, he sank his library to prevent humans from accessing his knowledge ever again. As the members of Team Avatar made their escape, Zei tried desperately to grab as many books as he could. Eventually, he simply declared that he would rather "spend an eternity" in the library than lose all of his life's work. Zei ended up dying in the library, inadvertently fulfilling his wish.
Despite his disappearance, Zei remained a known character among historians, and even Aang's granddaughter, Jinora, came to learn about Zei's story during her studies. Seventy years following the relocation of the library to the Spirit World, she arrived at Wan Shi Tong's Library, stating that she could spend her life reading in the building. Wan Shi Tong informed her that the last human who said that was still there, before motioning toward the skeletal remains of Professor Zei, slumped against a bookshelf and surrounded by books and scrolls.