Fire Sage Shyu was, at age sixty, the youngest of the five Fire Sages and was the last of his proud lineage who did not follow the Fire Lord. He firmly believed that once AvatarRoku returned to the sanctuary, all the sages would pay a price for serving Fire Lord Ozai, who had set forth a decree stating that anyone who aided the Avatar would be branded a traitor by the Fire Nation and handled accordingly. Although aware he would suffer for assisting the Avatar, he remained loyal and helped Aang communicate with Roku's spirit.
Shyu came from a proud lineage of Fire Sages. His grandfather, Kaja, was Avatar Roku's master during his Avatar training, and was later the first person to deny that the Avatar had been killed during the first assault on the Air Nomads. He passed this conviction on to Shyu's father, who became an outspoken critic of the Fire Lord's ambitions, and whose support for the Avatar was branded by the Fire Nation as sedition. After Shyu's father was executed for treason for continuing to profess his loyalty to the Avatar, Shyu was left an orphan. He was taken in and raised by the Fire Sages, though he never felt close to any of them. Despite firsthand knowledge of the penalty faced by defying the Fire Lord's orders, Shyu remained loyal to the Avatar.
Several weeks before Aang's arrival at Roku's temple, the eyes of the statue of Avatar Roku began to glow at the same time Aang entered the Avatar State in the Southern Air Temple, informing the Fire Sages of the Avatar's return. Unlike his colleagues, who obeyed the Fire Lord's will, Shyu chose to help the Avatar.
When the Sages attempted to capture Aang, Shyu assisted him by leading him through a secret passage that led to Roku's sanctuary. After a failed effort to open the locked chamber containing Avatar Roku's statue, Shyu aided Aang by tricking the other Sages into believing Aang had already opened the chamber. The Sages opened the door and Aang was able to enter before they could catch him; the chamber was sealed shut by a white light and would not open up until Roku let it. Shyu was consequently labeled a traitor for upholding the sages' original duty to protect the Avatar rather than remaining loyal to Ozai. When Aang emerged again, possessed by Avatar Roku, Shyu ordered Katara and Sokka to leave before Roku's spirit destroyed the temple.
After the temple's destruction and their escape, Shyu was among the five Fire Sages captured and imprisoned by Zhao. Although he was the only one disobedient to the Fire Nation's cause, his colleagues were also branded traitors by Zhao.