Lian

Lian, also referred to as "The Maker", was a highly skilled mechanist responsible for the creation of havoc-wreaking machines. Because the Fire Nation was about to win the War, and because she had no confidence in Aang's ability to master all the elements in time, she believed the only way to defeat the Fire Lord was to build her machines. Since she did not appreciate the bending arts, she concocted a plan to destroy the benders and replace them with her creations, preventing any future war.

History
At some point in her life, Lian was captured and locked away in prison by the Fire Nation, who wanted for her make machines for them. When Aang and Sokka realized Katara had gone missing, they journeyed to the Fire Nation prison in order to rescue her. They searched the jail, and upon entering the final room, they also reached Lian's cell. Sokka and Aang fought the jail guard to set her free; during this battle, Lian noticed that Aang was an Airbender, which gave her an idea. Once Sokka and Aang had weakened the jail guard, they found that Lian had escaped in the meantime, leaving only a map to an old Earthbender training camp.

Upon arrival to the camp, the group discovered that it was infested with small bending machines. In the training part of the island they had defeated the machines and Katara used her healing abilities to cure the injured fighters.

Eventually, the gang went a hidden island where they found Lian building machines. She revealed to them that she was making the machines to get rid of benders, because they were responsible for the War. She made Aang an offer: He could join her and her machine army or be killed off with the other benders. The Avatar declined this offer, seeing as how the machines were creating more problems than solutions.

Trivia

 * Lian's ultimate fate after Aang defeated the Ultimation differed in each version of the game. In the console version, it was implied that Lian was crushed when the Ultimation toppled over. In the PSP and DS version, Lian collapsed in disbelief and Yuan said that he will make sure that he and Lian atone for their actions. The Game Boy Advance version gave no indication of what happened to Lian.
 * She had what was called bender envy: She envied the capabilities of the benders, and thus made machines since she believed that bending was nothing more than "magic".
 * This "bender envy" displayed by Lian was strangely reminiscent of the anti-bender revolt that will occur in The Last Airbender: Legend of Korra.
 * Lian can be translated into 连, which means to connect or join.