Gyatso, also known as Monk Gyatso, was Aang's guardian before the 100 year War. He mentored Aang in Airbending and they both considered each other close friends. He was also a member of the Council of Elders.
Gyatso was a very kind and witty. Aang claims he was the greatest airbender ever. Gyatso was friends with Avatar Roku when he was younger.
He wasn't strict towards Aang when he was teaching him Airbending; he allowed Aang to have fun while training.
Sadly, he was killed by the Fire Nation when Fire Lord Sozin attacked the air temples during the first attack of the 100 Year War.