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 Hundred Year War, and 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 through the machines she built. 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 her to construct 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 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, Katara 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 arriving at 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. Unfortunately, a Bender, Haru's friend, has gone missing in the fray. Sokka points out that a similar situation happened in the North Pole with a Waterbender.

Eventually, the gang went to a hidden island where they found Lian building machines. She offers a chance to work with her to end the war, but Aang turns her down, stating that he could never be a part of something that hurts innocent people. She unleashes her prototype machine on him and his friends, which they defeat while she escapes to an Air Temple.

At the Air Temple, she releases her machines in the Sanctuary, trying to destroy the Avatar statues. Meanwhile, she has a large machine as well others near the construction site where Aang's friends as captured. It's possible that she tricked Aang into leaving his friends, making both them and him vunerable. Later, when Aang finds the Fortress, he discovers Lian's machines had cut off water to a nearby village and are running amuck. He also finds that she has put his friends in different areas of the Fortress and in "cages"  that they can't escape from.

Once the gang has defeated all of Lian's machines and the Dreadnought, they find her in a large room where they discover the Ultimation, a large machine that can "bend" all four elements and is equipped with various weapons. She revealed to them that she was making the machines to eliminate benders, because they were responsible for the war. She also reveals the three Benders she had captured that power her machine: Yuan, Haru's friend; a Fire Nation soldier and Hiryu, the Waterbender from the North Pole. She had threatened to kill them along with the other Benders if they didn't agree to help her. She made Aang a second offer, telling him they could end the war and become a full Avatar. The Avatar declined this offer, telling her there's much more than controlling all of the elements. Enraged, she declares her machine more of an Avatar than him and she doesn't need a mindless Bender like him. The gang battle it and seemingly defeat it, but when Sokka tends to Haru's injury and Aang's back is turned, it reactivates. Katara throws herself in front of it and gets injured in the process. Lian mocks Aang for thinking he could defeat her machine, but her words and Katara's sacrifise triggers the Avatar State. Aang uses it to defeat Ultimation, which falls on Lian and crushes her.

Trivia

 * Lian's ultimate fate differed in each version of the game. In the Xbox, PS2, NGC and Wii version it was implied she was crushed to death when her ultimate creation, a "mechanical avatar" (the final boss), was beaten and toppled over. In the PSP and DS version Lian collapsed in disbelief at the defeat of her ultimate creation; Yuan vowed that he and Lian would atone for their actions. The GBA version gave no indication of what happened to Lian. (Lian was not in the PC game.)
 * 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 bears similarities to the Anti-bending Revolution of the Equalists in The Legend of Korra.
 * One of the Equalists, Hiroshi Sato, created machines for similar purposes as Lian's.
 * The seal present on all of Lian's inventions bears similarities to the one on Asami's shoulder band and on other things made by Future Industries.
 * Also, Amon's greater speech is somewhat similar to Lian's, stating that bending was the source of every war in mankind, and once they were removed from society, there will be no war.
 * Lian can be translated as, meaning to connect or join.
 * All of Lian's machines are based on the four bending elements: The Soaker = Water, The Inferno = Fire, The Tempest = Air.