Monk Gyatso is a minor character of Avatar: The Last Airbender, glimpsed briefly as part of the central background of main character Aang.
Monk Gyatsp was an airbender of the Northern Air Temple's Council of Elders, who trained, tutored and centrally raised Aang in the monastery in the time period just prior to the One-Hundred Year War. Aang, has stated that Gyatso was the greatest airbender in the world of his time, though it is unclear if this is embellishment of his powers, fact or just a comment on his personality over that of his powers. Gyatso was however the main influence on Aang's training as an airbender so even with Aang's natural gift with bending Gyatso is largely responsible for Aang's prowess as an airbender.
Not only was Gyastos know for his kindness and wisdom but also for his humor. Gyatso emphasizes that balance is the most important aspect of training as an airbender, he takes this to it's logical conclusion by also focusing on balance for a mind-set for Aang. For every serious training session that Gyatso had with Aang about form and technique he also incorporated things like fun Pai-Sho, air-gliding and pie fights into such lessons to educate Aang on mechanic and exploits of his powers while keeping him cheerfully invested as the lessons as a form of fun. Gyatso had told Aang that he and Avatar Roku, Aang's previous incarnation, were very good friends. Despite his preference for non-violence, when his temple was attacked he slew a room full of comet-powered firebenders, before his own demise.
When Monk Gyatso was pulled aside by the Council to step up Aang's training with new mentors, Aang, who overheard the notion, could not take the pressure of the situation to say nothing of the grim-outlook of not being able to train with Gyatso anymore, who he had viewed as a father-figure. Aang flew off but was caught in a storm, he froze himself instinctively to try to survive it and would remain there for one hundred years. When Aang returns to the Southern Air-Temple with his friends, Katara and Sokka, Aang soon finds just how long he had been frozen by discovering Gyatso's corpse. Aang soon finds out from Katara and Sokka that the Lord of the Fire Nation had wiped out the airbenders when he heard the Avatar was reborn into the air nomads.