A master Airbender that was one of five monks that resided at the Southern Air Temple over 100 years ago, Monk Gyatso was Aang's guardian. He helped Aang with his airbending training, played games with him, and overall just made his time at the temple much better. Even though Aang was training to be the Avatar, he believed that Aang should grow up as a normal boy, despite the other monks' decisions. Monk Gyatso was highly respected, was a great baker, and was kind and very wise. He was like a father to Aang, and they got along together very well.
Although the details are fuzzy, when the Fire Nation found a way inside the Southern Air Temple, they managed to kill Monk Gyatso and perhaps the other monks and airbending boys as well. Gyatso did, however, go down fighting and took many firebenders with him. A statue in his honor was placed in the entrance hall to the Southern Air Temple's inner sanctuary, and is still there.