Appa

History
The earliest known stage of Appa's life is when he was a young calf. This is the moment when he first encounters Aang. It appears Appa's mother was willingly directing her calves down to the young monks, with whom they will become companions for life. Not much is explained about the exact nature of the relationship between the Air Nomads and their flying bison. Aang came forward and offered an apple to Appa. Almost immediately, the two became friends and would be together for most of their lives up until the present day.

Appa was stuck in suspended animation along with Aang for 100 years until their discovery by Katara and Sokka. Since then Appa has been the group's main form of transport in their quest to help Aang master all the elements. Appa was a consistent part of the group throughout the series until his capture by the sandbenders.

Appa became lost for a short while, when the group was traveling in the Earth Kingdom. He was kidnapped by Sandbenders in the middle of the vast Si Wong Desert, while Aang, Katara, Sokka, and Momo were trapped in a vast underground library. It is later revealed that he was eventually sold to a Fire Nation circus where a sadistic animal tamer attempted to make him part of the show. Appa later escaped and journeyed throughout the Earth Kingdom until he encountered the Kyoshi Warriors, who attempted to heal the various wounds he had sustained in his travels and escort him back to Aang. Appa was forced to leave after an attack by Azula and her allies, ushered away by Suki while the warriors tried to delay the three Fire Nation women. On his journey he encountered Guru Pathik at the Eastern Air Temple. The Guru was able to tell Appa of Aang's location by reading his energy. Consequently, Appa flew to Ba Sing Se, where he was captured by Long Feng.

Appa was kept in a secret location at Lake Laogai until his rescue by Zuko, who was initially planning on utilizing him to capture the Avatar. Appa eventually returns to the main group in the last scenes of "Lake Laogai". Aang is particularly overjoyed to see his old companion again.

Now as they travel through the Fire Nation, Appa must stay hidden. When Appa flies, Aang and Katara must create a cloud around him so that to civilians, he just looks like a cloud. Also, when they leave him and Momo alone, they must cover him a some sort of camouflage.

Personality
Little can be said about Appa's personality, as he lacks the ability to speak. However, he has displayed an ability to understand humans on several occasions. He is also afraid of going underground and of fire. His fear of fire could be due to his experience at the circus or a natural tendency of his species. It would be safe to assume that Appa had a reasonably trusting attitude before the events of "Appa's Lost Days". He did not take long to accept the apple offered to him by Aang and took an immediate liking to Sokka in the first episode of the series. However, this trait was damaged due to his experience with the ringmaster.

Abilities
Appa possesses the ability to fly, hence the name Flying Bison. He utilizes his Airbending for this purpose; his tail appears to be the main driving force behind his bending but has shown to also be able to accomplish it with his mouth. Due to his size and mass, it takes a lot of energy for him to stay aloft. As a result, Appa cannot fly for an extended time and periodically requires stopping and resting. He has also shown to be a fairly adept swimmer as well, although he is more frequently seen flying or walking than swimming. Despite the tranquilty related to his personality as well as his species, Appa has shown himself to be very capable of fighting when necessary. With his bending, Appa can create powerful gusts of air with just a flick of his tail and can gain advantage over his oppenents by hovering. Being a bison, he is also naturally capable of brute strength and endurance. The best demonstration of this was in "Bato of the Water Tribe" and "Appa's Lost Days".

Aang, in "The Painted Lady," also remarks the fact that Appa has five stomachs that might be because of his size and the amount of energy he needs to fly.