Even 15 years later, Avatar: The Last Airbender still remains one of pop culture's biggest and most popular franchises. It's (and by extension it's sequel/spin-off The Legend of Korra's) overwhelming success with critics and audiences alike extend to not just its technical achievements, but also it's story and characters which widely praised as some of the best in the genre.
Outside the main cast you'll find a plethora of side characters all with their own histories, personalities, and memorability. These characters may not be a part of Team Avatar or even lead characters, but they pull their own weight and are memorable for a reason.
Jet
One of Aang's earliest adventures found himself and the group teaming up with a freedom fighter by the name of Jet. With his home having been destroyed by the Fire Nation, Jet has made it his mission to rid his region of firebenders, any means necessary. This puts him into conflict with the group when they feel he's going too far by completely wiping out a Earth Kingdom village the Fire Nation just happens to be in.
Jet later returns in a more significant role in Season 2 where he ends up involved with the Dai Li conspiracy, and sacrifices himself to protect the group from Long Feng. Jet's complexity as a character is what makes him engaging to watch, and while he isn't around long he showcases how people have been affected negatively by the war and the trauma that it has put people through.
Jinora
One of Tenzin's four children and his oldest, Jinora takes her status as an airbender much more seriously then either of her siblings. She becomes the youngest airbending master since Aang at 11 and has a strong connection with the spirits and the Spirit World at large, something which she finds affinity with in Korra.
Jinora's strength comes from her embodying the tenants of the Air Nomads. While Tenzin may have be pushed by Aang to follow in his footsteps, in reality it's Jinora who is the future of the airbending legacy. Her strength comes from her courage and her kindness, and those are the teachings that Monk Gyastso passed down to Aang.
Kai
A young orphan from Ba Sing Se that received airbending because of the Harmonic Convergence, Kai should be everything the airbenders aren't. He's opportunistic, cunning, and ultimately tries looking out for himself over others due to his upbringing, which makes it a funny coincidence that he knows airbending.
However, as time goes on, and as he integrates more into the newly formed Air Nation, he ends up fitting in quite well - and not just because he has a crush on Jinora. As it turns out, he's in reality a compassionate, empathic young kid who just wants to protect the things closest to him. This allows him to become one of the Air Nation's premiere airbenders and one of their more spiritual members.
King Bumi
An old friend of Aang's prior to him going into the iceberg for 100 years, Bumi at some point becomes the king of the city of Omashu and one of the most powerful earthbenders in the entire world. While initially Aang planned him to be his earthbending teacher, Bumi's own duties and his capture at the hands of the Fire Nation prevented that from happening.
Bumi is a odd fellow for sure. He speaks in non sequiturs and clearly has out of the box thinking, but it's that out of the box thinking that makes him so dangerous. Bumi's earthbending skills are only really matched by Toph, and even then when Toph and Bumi eventually did duel it ended in a draw.
Lin Beifong
Toph's daughter and the head of the Republic City police force, Lin is a powerful metalbender like her mother and just as no-nonsense - albeit without the sense of sarcasm this time. While he's gruff and tends to keep to herself, Lin is dedicated to her job and genuinely wishes to help the people of Republic City despite corruption within the higher reaches of government.
Her relationship to the cast is one of the more interesting dynamic in The Legend of Korra, as we see that Tenzin and her dated for a while even if it didn't go anywhere, and her strained opinion on Korra which evolves over the course of the show. Once her sister Suyin is introduced her role becomes ever more fascinating, as we see the results of Toph being a somewhat lackluster parent take root.
Mai
One of Azula's allies during the time she was chasing Aang, Mai is complicated individual. Having been born a spoiled rich girl with very little time to herself, Mai finds boredom in mostly everything around her. And while her inability to connect with people is what drives her to join Azula, she does end up connecting with Zuko, whose own complicated relationship with his parents made them relate to each other.
Mai would later turn her back on Azula and fully join up with Zuko after the war, though their relationship ended in the comic Smoke and Shadow after some disagreements. There is still a chance, however, that Mai could get back together with Zuko in the future, as the stories of Aang and his group in the comics is yet to be finished.
Master Piandao
A master swordsman and swordsmith, Piandao first appeared in the episode "Sokka's Master" as somebody to help train Sokka in the ways of the sword. While he's able to pick up on Sokka's identity pretty quickly, he trains him anyways, believing the art of the sword to be a universal teaching and not just for any one nation.
While he disappears soon after that, he returns briefly in the finale as a member of the White Lotus to reclaim Ba Sing Se. His effect on Sokka is also massive, as it changes his character into a more mature, battle hardened character that feels more intregral for the conflicts to come.
The Cabbage Merchant
Even if he doesn't have a name, this fan-favourite background character is worth mentioning regardless. All he wants is to sell his cabbages and earn his living, but the world just seems to not want him to do that. Whether it be in Omashu or Ba Sing Se, misfortune seemed to follow him around - and Aang at points was indirectly responsible, much to his chagrin.
He gets a bigger role in the comic trilogy The Rift, where he eventually shifts his focus from cabbages to technology. This eventually leads him to create Cabbage Corp, a technological powerhouse focused on the creation of automobiles. But even then, misfortune seemed to hit the company, as it was found making weapons for the Equalizer movement and was nearly put out of business. He just can't catch a break, can he?
Ty Lee
Azula's self-proclaimed best friend and a master of chi-blocking, Ty Lee may be bubbly, happy, and a bit naive, but she is also a force to be reckoned with. Benders beware - her chi-blocking will put a stop to your abilities real fast.
Beyond that though, Ty Lee, like the rest of the Fire Nation kids, has a lot of baggage surrounding her. One of 7 identical sisters, she ended up joining the circus find her own path, and after finally rejecting Azula's control of her joins the Kyoshi Warriors, teaching them about chi blocking as well.
Varrick
In whom can only be described as the Howard Hughes of the Avatar universe, Varrick is an inventor, billionaire, and venture capitalist who is always looking for the next great business opportunity. Whether it be against Korra or working with her, Varrick can be untrustworthy, but he always knows a good sell when he sees one and is always found with his trusty assistant, Zhu Li.
At least, until Book 4, where Zhu Li finally gets fed up with Varrick's behavior. This causes Varrick to have a crisis of conscience, realizing that his technology has been used for terrible things. In a character turn that is extremely similar to that of Tony Stark, Varrick decides to dedicated himself to making energy sources and making the world a better place. Zhu Li comes back as well, and eventually the two of them wed in the South Pole on the same day that Korra and Asami start a relationship themselves.