Appa first met Aang at the Eastern Air Temple while still a young calf. Young airbenders, usually six years old, were allowed to meet their future companions, who would bond with them for life. Not much was explained about the exact nature of the relationship between the Air Nomads and their flying bison, but it appeared that the herd's mother willingly directed her calves to meet the young monks.
Aang came forward and offered an apple to Appa, and almost immediately afterward, the two became best friends and were together for most of their lives up until Aang's death.
Journey to the North
Just before the Air Nomad Genocide, Aang heard that the monks were going to separate him from MonkGyatso, and he subsequently decided to run away, using Appa as his mode of departure. Appa did not know where he was going, only that he was leaving the air temples. Aang flew Appa over the ocean, at which point they were caught in a vicious storm which pushed the two underwater by stirring up the wind and the waves, and Aang lost consciousness. Faced with a life-or-death situation, the young Avatar entered the Avatar State and froze himself and Appa in an iceberg, where Appa was stuck in hibernation along with Aang for one hundred years until they were discovered by Katara and Sokka. After this, Appa became the group's main form of transportation in their quest to help Aang master all four elements.
Appa became lost for a short while when the group was traveling through the Earth Kingdom, having been kidnapped by sandbenders in the middle of the vast Si Wong Desert, while Aang, Katara, Sokka, and Momo were trapped in an underground Spirit library. It was later revealed that he was eventually sold to beetle-headed merchants, who in turn sold him to a Fire Nationcircus where a sadistic trainer attempted to make him part of the show. Encouraged by a sympathetic boy, Appa later escaped and hurried back to the Si Wong Desert where he last saw Aang. Understanding that his friends were not there anymore, he journeyed throughout the Earth Kingdom until he encountered the Kyoshi Warriors, who healed his various wounds, physical as well as emotional, that he had sustained on his travels. They wanted to escort him back to Aang, but they were soon thwarted in their attempt when Azula and her allies attacked. Appa was forced to flee, ushered away by Suki while the warriors tried to delay the three Fire Nation girls. On his journey he encountered Guru Pathik at the Eastern Air Temple, where 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 under Lake Laogai until his rescue at the hands of Zuko, who was initially planning on utilizing him to capture the Avatar, but after a talk with his Uncle Iroh, he decided to do the right thing and set the bison free. Appa found the group cornered by Long Feng and the Dai Li and was instrumental in their defeat, biting Long Feng on the leg when confronted by him. Once safe, Aang welcomed Appa in a tearful display of affection.
Appa took part in forcibly entering the Royal Palace of Ba Sing Se, providing the first piece of evidence to KingKuei of Long Feng's subterfuge since Long Feng's bite mark still lingered. When the Dai Li's cover up efforts left the Earth King skeptical of Team Avatar's claims, Sokka made a last-ditch effort to show the king the ruined Fire Nation drill by enticing him with a ride on Appa. Since Appa's saddle was lost during his capture, the group was forced to ride bareback. After the fall of Ba Sing Se, Appa was seen spiriting Team Avatar, including a near-fatally wounded Aang, the Earth King, and his pet, Bosco, out of the now-captured city.
In the Fire Nation
Appa stayed with the group as they found their way aboard Hakoda's commandeered Fire Navy vessel and had a new saddle constructed for him. After Aang ran off into a storm, Appa and the rest of Team Avatar split with Hakoda's crew and chased down Aang, and they regrouped on Crescent Island. As they traveled through the Fire Nation, Appa was forced to stay hidden, and when Appa did have to fly, Aang and Katara created a cloud around him so that he resembled a cloud to any Fire Nation civilians. When they left him and Momo alone, the gang had to cover Appa up with some form of camouflage.
After the invasion of the Fire Nation failed on the Day of Black Sun, Appa took Aang's friends and all of the younger members of the invasion force to the Western Air Temple in search of refuge. Unbeknownst to Team Avatar, Zuko, who had abandoned the Fire Nation during the invasion, had followed them to the temple, and when he arrived, Appa immediately welcomed and licked him, a direct result of Appa remaining grateful to Zuko for freeing him from his imprisonment in Ba Sing Se. Zuko requested that he join the group so that he could teach Aang firebending. Despite Appa's obvious welcoming of Zuko, the group did not accept Zuko until later, when he saved the group from an attack conducted by Combustion Man.
In the final days of the Hundred Year War, Sozin's Comet's arrival, Appa helped Team Avatar, excluding Aang, travel to the Earth Kingdom, where they had June and her shirshu, Nyla, try to track Aang. Unsuccessful, they instead tracked down Iroh, using his sandal, who was residing outside Ba Sing Se with the rest of the Order of the White Lotus. Appa later escorted Zuko and Katara to the Fire Nation Capital for their confrontation with Azula.
A year later, when the movement was being carried out, Appa helped Team Avatar transport Fire Nation colonials back to their nation.
One year following the failure of the Harmony Restoration Movement, Appa once again embarked on a mission and transported Team Avatar to Hira'a in search of Ursa. However, he was visibly upset that Azula would be accompanying the team on the trip. On their way there, the party was attacked by a wolf spirit, and he aided in the battle against the beast. Once the team learned about Ikem's flight up Forgetful Valley, Appa stayed behind and later flew back to Hira'a along with the remaining members of Team Avatar and the Mother of Faces.
There was little said about Appa's personality, as he lacked the ability to speak human language. However, he had displayed an ability to understand human language on several occasions. He was also afraid of going underground and, since his experience in the Fire Nation circus, of fire. Appa seemed to have a reasonably trusting attitude, as he did not take long to accept the apple offered to him by Aang and took an immediate liking to Sokka when he first saw him. However, this trait was damaged due to his experience with the ringmaster and the sandbenders. In these difficult moments, Appa seemed to have similar feelings toward humans when he missed Aang and he tried his best to find the team. In the Eastern Air Temple, he also remembered with sorrow his childhood with his siblings and his mother.
Appa also might have had some degree of intelligence as shown when he yawned after Aang prompted him, and when he apparently "scolded" Hawky and Momo when the two fought due to Sokka's order that Appa was in charge. Appa's intelligence was also shown when he was taking the team and Professor Zei to Wan Shi Tong's library. Zei asked Appa a question and Appa tried to respond, only letting out a groan.
Being a sky bison, Appa possessed the ability to fly. He utilized his airbending for this purpose; his tail appeared to be the main driving force behind his bending, but he also had shown the ability to accomplish it with his mouth. Despite the tranquility related to his personality as well as his species, Appa had shown himself to be very capable of fighting when necessary. With his bending, Appa could create powerful gusts of air with just a flick of his tail and could gain an advantage over his opponents by hovering. He even defeated Long Feng, Ty Lee, and Mai.
Due to his size and mass, it took a lot of energy for him to stay aloft. As a result, Appa could not fly for an extended time and periodically required stopping and resting. However, there were times when he displayed extreme stamina, such as when Katara stated that they flew two days non-stop, as well as when Appa continually had to fly to avoid Azula, Ty Lee, and Mai for hours on end with only a few minutes of rest each time he stopped. He was also capable of carrying quite a burden on his back, having once carried seven medium-weight people in one journey.
Being a bison, he had a great deal of brute strength and endurance. The best demonstrations of this were in the fight against Nyla in the abbey, and against a fully-grown boar-q-pine in the forest. His strength was so great to the extent of Appa lifting two Fire Nation tundra tanks simultaneously each by the horn, flipping them over. Using his teeth alone, he was capable of grabbing Long Feng by his leg and throwing him a great distance.
Appa was also capable of swimming, even being a fairly adept swimmer, however, he would more frequently fly or walk rather than swim.
Appa was an herbivore, as indicated by his flat teeth and diet, which was mainly composed of plants and fruits. Like most farm animals, Appa seemed to prefer hay when it was available. Appa also consumed honey and attempted to snatch an egg when he was desperate for food.
Appa would not respond to the bison whistle if he was napping.
According to his missing posters and a statement made by Aang, Appa's weight was ten tons. In "The Warriors of Kyoshi", Sokka referred to Appa as a "ten-ton magical monster".
Appa's design is similar to that of the Catbus from the anime film My Neighbor Totoro which was made in 1988. The Catbus also has multiple legs and a large tail.
Co-creator Bryan Konietzko described Appa as a manatee-bison hybrid, although the manatee aspect of Appa is extremely subtle. Early development sketches of Appa show him with much more manatee-like facial features.
Early designs also show Appa with spiral horns, however they were removed due to being "too difficult to animate by hand".
Appa was originally a dog-like animal and much smaller, albeit still larger than Aang. The design was eventually recycled into Naga, Korra's animal companion.
Appa was claustrophobic, and after the events in the Fire Nation circus, he became pyrophobic as well.
It was shown that Appa could sleep on his side with his legs wrapped around his tail.
Although Aang is said to be the last airbender in the series, technically with Appa being a sky bison he was an airbender too. This means more specifically that Aang was the last human airbender and more so the last Air Nomad.