The Avatar before Aang was Roku of the Fire Nation, and his role in the storyline cannot be underestimated by any means. He spends his life traveling the world, learning as much as he can — of bending disciplines, of diplomacy, of culture — in order to become an ideal mediator between the Four Nations.
Unfortunately, Roku's narrative does not end well: his childhood best friend, Sozin, leaves him to die so he can pursue his own greed. However, this Avatar's influence extends far beyond his death; he appears regularly in spirit form to guide his successor on the right path. And where Roku could not succeed, his "student" does.
10 Member Of The Fire Nation Royal Family
Roku's earliest years are spent in the Fire Nation Palace, together with Sozin. The nature and strength of their friendship, as well as the fact that he is allowed free access to the Royal household, suggest that the Avatar is likely of royal blood himself.
It's not clearly explained in the storyline, but given that he and Sozin celebrate their birthday together (which is quite a big deal when the Crown Prince is sharing his special moment), it's quite believable.
9 Bending Training
Roku learns the other elements in the correct order. Airbending comes at the beginning, which he studies at the Southern Air Temple under the tutelage of the Monks.
Waterbending turns out to be a lot more complicated, as it counters his natural element, but Roku succeeds at this as well (by throwing his sifu halfway across the Northern Water Tribe capital city.) Finally, he picks up Earthbending; similar to the previous experience, he proves his talents by surpassing his teacher in a rock race.
8 Relationship With Sud & Gyatso
Roku's Earthbending master, Sud, remains one of his closest friends and confidantes long after he trains him. The Avatar may have referred to him as "uncompromising, stubborn, and blunt," but this merely reflects Sud's inherent nature as an Earthbender.
Interestingly, this raises the possibility that Roku maintains a bond with all of his bending teachers, since he reveals to Aang that he and Monk Gyatso had been pals for several decades.
7 The Avatar State
At the age of 27, Roku travels to the Crescent Island Fire Temple, where he tries to figure out the various complexities of the Avatar State. He receives mentorship from the famed Kaja, an extremely skilled Firebender and one of the five Fire Sages.
Although Roku initially loses sight of his goal and begins unconsciously destroying the area, it's Kaja who helps him regain his inner balance. Interestingly, the Fire Sage Shyu who helps Aang enter Roku's sanctuary happens to be Kaja's son.
6 Destroying The Fire Temple (Twice)
As mentioned above, Roku basically decimates a large section of the Fire Temple during one of his attempts at the Avatar State, for which he apologizes by refurbishing the building. He apparently restores the temple as well as creates subterranean tunnels with the help of Lavabending.
The second time, however, is seemingly permanent — to protect Team Avatar as well as punish the Fire Sages for failing their duties, Roku employs Lavabending to melt the entire place to the ground.
5 His Wife And Children
His training complete, Roku returns to the Fire Nation and once again meets with his beloved childhood friend, currently the Fire Lord Sozin. He proposes to Ta Min, and they have a magnificent wedding ceremony and banquet, presumably inviting dignitaries from all over the world.
Their only child, Rina, later marries a man named Jinzuk in a village known as Hira'a, where she gives birth to a daughter, Ursa: she is consequently forced to marry Ozai by Fire Lord Azulon, under the belief that the Avatar's genes would be in his future heirs.
4 His Discovery Of Fang
Avatars have a penchant for spirit animals (Aang has Appa, the Sky Bison, and Korra has Naga, the Polar Bear Dog.) Likewise, during Roku's era, dragons still roam the earth free from any threats, at least until Fire Lord Sozin begins poaching them for fun.
Roku discovers a tiny egg without any other adult in sight, so he assumes that the dragon within is either orphaned or lost. In any case, he adopts the creature when it hatches, and it stays with him for the rest of their lives.
3 Temporarily Loses Contact With Aang
After the Hundred Year War, Aang finds himself in the middle of a political conflict with Fire Lord Zuko, so he naturally "visits" Roku for guidance. As usual, his predecessor insists that he murder Zuko in order to preserve global harmony, which the young Avatar refuses to do.
When Roku scolds him for "showing mercy," Aang angrily destroys his fire amulet, thereby cutting off all ties with the former. Sadly, he learns that this act also breaks the link with the whole chain of Avatars, so he revives his medallion, thereby bringing Roku back once more.
2 Has A Preference For Airbending
Roku's true spirit revolves around Firebending, but from his actions during the narrative, it looks like he has a slight bias for the unfettered art of Airbending.
When Sozin audaciously challenges him, he resorts to Earthbending to avoid the former's attack, but the only form of offense used is wind-based. Roku smashes open part of the Royal Palace with an overwhelming blast of Airbending. Before his death, he also uses the same style to congeal the emerging lava, as well as to divert the streams of burning, poisonous gas bursting out of his home volcano.
1 Why Couldn't He Airbend The Volcanic Gas Out Of His Lungs?
Sozin ignores Roku's plaintive cries for help, as the latter slowly chokes to death after accidentally inhaling a cloud of toxic volcanic fumes. Instead of using Airbending to extract the harmful gases out of his body, the Avatar resigns himself to his fate and curls up to die, along with Fang.
In The Legend of Korra, Zaheer proves that it's possible to excavate all the air from one's lungs, which is what he does to Earth Queen Hou Ting. In any case, Roku's failure results in the Fire Nation mounting its Hundred Year War.