"With Roku gone and the great comet returning, the timing was perfect to change the world. I knew the next Avatar would be born an Air Nomad, so I wiped out the air temples. But somehow, the new Avatar eluded me. I wasted the remainder of my life searching in vain. I know he's hiding out there somewhere. The Fire Nation's greatest threat... the last airbender."
As a young prince, Sozin was best friends with AvatarRoku. After Sozin took up the throne and Roku became a Fully Realized Avatar, their friendship became strained to the breaking point as Sozin desired the Fire Nation to encompass the world, a plan to which Roku was strongly opposed. Following a confrontation over this which ended in Sozin's defeat, neither spoke to the other until they were both very old. Sozin betrayed his old friend to his death and began waging war.
In his youth, Prince Sozin and Avatar Roku were best friends and even shared the same birthday, which they celebrated together. It could be said that both of them were also firebending masters at an early age, given that Roku was the Avatar, which was then unknown, and Sozin's family had a natural affinity for their element.
At their sixteenth birthday celebration, Roku was unexpectedly told by the Fire Sages that he was the Avatar, and the two friends were forced to separate as Roku went to learn mastery of the three other elements. Before leaving, Sozin came by to talk and was told by Roku that the Sages told him to bring nothing, as the Avatar did not need material goods. By this statement, Sozin gave him a gift: the headpiece worn by the Crown Prince of the Fire Nation, hoping that he was at least allowed to have that. Eight years later, Sozin would ascend to the throne; following the death of his father.
Twelve years later, Roku returned to the Fire Nation. At Roku's wedding, Sozin took the position of his best man. At the wedding, Sozin commented that the Fire Nation was going through a period of great prosperity and wealth and told Roku that together, the two of them could spread it throughout the world by expanding the empire. Roku was greatly disturbed by this, telling Sozin that the four nations were meant to be separate and that he should not think of this again.
Several years later, however, Roku discovered that Sozin had set up colonies in the Earth Kingdom and came to rebuke him, leading Sozin to attack him. The two engaged in a battle, with Roku finally destroying most of the palace and defeating Sozin. Roku spared his life because of their past friendship, but warned him never to move forward with his plans again, or it would result in his "permanent end".
Twenty-five years later, Roku's home island was consumed in a volcanic eruption so massive that Sozin could see and feel it from a hundred miles away. The Fire Lord flew in on his dragon to assist his old friend. The two used their bending to try to contain the lava flow, but were hindered by poisonous gases, which eventually overwhelmed Roku, leaving him lying on the ground and begging for his old friend's help. Sozin suddenly realized, however, that Roku's death would allow him to fulfill his plans, and left his old friend to die along with his dragon, Fang.
Twelve years after Roku died, Sozin used the power of a comet to enhance the abilities of his firebender army and launched a genocidal attack on the Air Nomads in an attempt to kill Aang, the new Avatar, and halt the Avatar Cycle. However, Sozin failed, as Aang had run away from the air temples some time before. Sozin spent the rest of his life searching in vain for the last airbender. After the comet, he lived twenty more years and had a son, Prince Azulon.
One of his lesser-known military accomplishments during this period was the Battle of Han Tui, which was applauded as a magnificent display of skill and military cunning. During his reign, Sozin started the tradition of hunting dragons for glory. If a firebender proved able to kill a dragon, the original firebenders, he would be showered with glory and praise, and earn the honorary title "Dragon". This led to the near-extinction of the dragons.
He eventually died at the age of 102. Rumors circulated that he lived to be older due to the influence of the power of the comet. According to what was learned in Fire Nation schools, Sozin died peacefully in his sleep, a very old and successful man. However, it appeared that at the end of his life he regretted what he started through his statement, "As I feel my own life dimming, I can't help but think of a time when everything was so much brighter..."
The War that Sozin began finally ended in 100 ASC when Avatar Aang, the very person he had tried to hunt down, re-emerged after a century of suspended animation and defeated Sozin's grandson, Fire Lord Ozai.
Sozin was shown to be very ruthless as Fire Lord, as demonstrated by his ordering the genocide of the Air Nomads simply to locate and destroy a single obstacle to his power, and his decision to start the tradition of hunting dragons despite having one as a companion. His betrayal of his closest friend, Avatar Roku, also highlights his willingness to embrace power over friendship and kindness. Unlike his even more militaristic son Azulon, however, Sozin (at least initially) saw his war with the other nations as a case of 'ends justifying means'- he hoped not only to increase his own power, but to provide the other nations' with the Fire Nation's higher standard of living and perceived better system of order. When he initially set up colonies in the Earth Kingdom, Sozin may have used military force against the Earth Kingdom Army, but he did not use violence or intimidation against the civilians. Although he ultimately let his own militarism overrun him, Sozin seemed to have a more human personality than his son; certainly, he cared more about his public image as a benevolent ruler. He was close to his family and for at least the first few decades of his life, treasured his friendship with Roku- a feeling that was reciprocated. Azulon, by contrast, was indifferent towards everyone except, to some degree, his eldest son Iroh. When Sozin saw Roku's volcanic island erupt, despite their unpleasant split decades earlier he went to help his former friend- though despite Roku saving him at one point, he ultimately left Roku to die once the civilians on the island were safe. However, in his twilight years, Sozin seemed to show at least some regret for the loss of his great friend.
Being Fire Lord, Sozin was a powerful firebending master. When he was young, he sparred with Avatar Roku and defeated him on at least one occasion. When Roku confronted him years later about his actions in the Earth Kingdom, Sozin attacked him by unleashing a massive wave of fire against him, even though he did not have access to the power Sozin's Comet provided. However, he was still no match for Roku, who had all four elements and the Avatar State at his command. When he helped Roku tame an erupting volcano, he used a technique to redirect heat, cooling the lava. The technique he used appeared similar to Iroh's lightning redirection move.
Sozin was a very intelligent strategist. When he and Roku were teenagers, Sozin tricked Roku into tripping over a tree root in order to win a sparring match. Sozin's intelligence made him a capable military leader. During the Hundred Year War, Sozin won the Battle of Han Tui by taking advantage of the fact that the enemy had been in the middle of a drought and downwind. According to Azula in a later recounting of the event, their defenses burned to a crisp in minutes.
Sozin's name, (蘇進), translates as "revival and progress".
Sozin's name is closely related to the Persian name Souzan, which means "fire".
Sozin's name is spelled very similar to Yamana Sōzen's surname. Both had disputes with another person, Sozin with Roku and Yamana with Hosokawa Katsumoto, which assisted in the creation of a war for both, although unlike Yamana and Hosokawa, Sozin and Roku were never actually at war against each other.
Fire Lord Sozin had his son, Fire Lord Azulon, very late in life; since Azulon was born shortly after the start of the War, Sozin would have been eighty-two years old at the time.
Sozin's official portrait features firebending soldiers, a scroll in his hand, and a comet, signifying his military buildup and the beginning of the Hundred Year War, the many reforms under his rule, and the coming of the comet which now bears his name.
Sozin appeared to have made the biggest impact of any character on the world, single-handedly starting the Hundred Year War and ordering the genocide of the Air Nomads.
An early script of the premiere episode included a friendly duel between Avatar Roku and Sozin when they were younger, similar to the one shown in "The Avatar and the Fire Lord".
People in the Avatar universe believe that the power granted from Sozin's Comet is what allowed Sozin to live as long as he did.
Sozin was the only known political figure to force an Avatar to choose between loyalty and duty and get away with it, albeit being threatened and having to wait until Roku died.
Sozin was also the Fire Lord responsible for manipulating firebending's roots, changing it to match his militaristic and conquering goals.
Sozin is the only character who has shown the ability to redirect heat.
Sozin began the tradition of hunting dragons, the original firebenders.
Sozin is the earliest Fire Lord developed in the series. He is also the oldest Fire Lord on record.
Technically, when his great-grandson, Zuko, and Avatar Aang established the United Republic, Sozin's ambition of uniting the people of all four nations under one rule, though misguided, was finally realized.