Sun was a young lavabender who lived in Yu Dao. After participating in underground bending matches, he was encouraged to join the Beifong Metalbending Academy as a student of its founder, Toph Beifong.
Sun lived in the former Fire Nation colony Yu Dao, and was part of group of street children. He and his comrades were suspicious of the Avatar, the Earth Kingdom government and its soldiers. At some point, Sun became capable of lavabending, although he could not fully control this ability, and it would sometimes suddenly manifest without him intending it to. He began to enter underground bending matches, as these provided an opportunity to get some money to support his group.
However, the tremors of his lavabending caused Toph Beifong to sense him from across the city during one match. Toph arrived to see Sun dueling an older earthbender, who began to mock him for his size and youth, calling him "pipsqueak". Sun responded by bending cracks of lava in her direction, telling her to remember his name. At that moment, a spectator noticed Toph in the audience, telling the rest of the crowd to leave, as he believed she had come to shut the match down on behalf of the authorities.
Sun returned to his matches the following night. Toph felt the vibrations once more and came to the top of the roof, where Chong helped her observed him. Sun's friends tried to discourage him from the brawls, but he told them that he would win enough money to get them by for months if he won the fight. His friend told him to not lose control, but he responded that he would be careful. He first dueled an older earthbender, who began to pummel Sun to the ground with rock gloves, but the lavabender got to his feet and sent a rock glove hurling in the other man's face, sending him to the ground and winning the match.
His next match was against a hugely muscular waterbender. Sun's opponent told him that he would surely defeat him, but the youth brushed off the intimidation. He shielded himself with an earth wall against a torrent of water bent from a barrel, which also held against the man's ice spikes. He sent a projectile block in the man's direction, but his opponent split it apart with his waterbending. Sun was caught off guard and began to slip on the wet floor, but as he stabilized himself on the ground, he sent a torrent of lava in the man's direction. The opponent initially believed that Sun had firebent, but Toph noted that it was in fact, lavabending.
Although most of the lava cooled on the ground fairly quickly, some of it hit the metal beams that were supporting the abandoned house and caused them to melt. The house began to fall apart, but Penga rushed to the scene and started to hold down the metal beams. Sun tried to metalbend when a panel came flying in his direction, and believed he had successfully metalbent when the panel levitated, but Toph told him that she had done it instead.
Sun and his friends fled outside, where he was apprehended by Toph wrapping him up in strips of metal. His friends demanded he be let go, but Toph wished to address him causing a house to collapse. His friends protested that he could not control his ability, but she began to make conversation with him. He apologized, telling her that he just wanted money, but Toph told him that staying alive was more important. Sun raised that tournaments were the only way to make money, but instead, Toph invited Sun and his friends to join the Beifong Metalbending Academy. Even though she knew not all of Sun's friends were metalbenders, she told them they could still perfect their skill at earthbending.
Sun expressed his distrust of Toph, as she was a friend of the Avatar, who was an ally to the Earth King and his military. Toph responded that although it was Aang's job to meet with world leaders, her own job was to keep her students safe and instruct them to become master benders. She also explained that she was the first person in a long time to discover a new form of bending, and that she wanted to teach someone with a rare and special ability like lavabending. Toph also introduced the other teachers: Ho Tun, Penga, and The Dark One. She explained that they would never have to sleep outside if they joined the academy, and that if they did not like it, they would always be free to leave. Hearing these words, Sun agreed to join the school, which pleased Toph.
- Sun is the third known non-Avatar to be capable of lavabending in accordance to the story's release order, with Ghazan and Bolin being first and second respectively. However, he is the earliest in accordance with the in-universe chronology.
- Sun was voiced by Olivia Hack when she performed a Dark Horse live reading of Toph Beifong's Metalbending Academy.