Lian, also referred to as "The Maker", was a highly skilled mechanist responsible for the creation of havoc-wreaking machines. With the Fire Nation about to win the Hundred Year War, and having 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.
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. However, a bender, Haru's friend, had gone missing in the fray. Sokka pointed out that a similar situation happened in the North Pole with a waterbender. Haru joined them to find his friend.
Eventually, the gang went to a hidden island where they found Lian building machines. She offered a chance to work with her to end the war, but Aang turned her down, stating that he could never be a part of something that hurt innocent people. She unleashed her prototype machine on him and his friends, which they defeated while she escaped to an air temple.
At the air temple, she released her machines in the sanctuary, trying to destroy the Avatar statues. Meanwhile, she had a large machine as well others near the construction site where Aang's friends were captured. Later, when Aang found the Fortress, he discovered Lian's machines had cut off water to a nearby village and were running amok. He also found that she had put his friends in different areas of the Fortress and in "cages" from which they could not escape.
Once the gang had defeated all of Lian's machines and the Dreadnought, they found her in a large room where they discovered the Ultimation, a large machine that could "bend" all four elements and was 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 revealed the three benders she had captured that powered her machine: Haru's friend, Yuan, 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 did not 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 was much more than controlling all of the elements. Enraged, she declared her machine more of an Avatar than him and she did not need a mindless bender like him. The gang battled and seemingly defeated it, but when Sokka tended to Haru's injury and Aang's back was turned, it reactivated. Katara threw herself in front of it and got injured in the process. Lian mocked Aang for thinking he could defeat her machine, but her words and Katara's sacrifice triggered the Avatar State. Aang used this power to defeat Ultimation, which fell on Lian and crushed her.
Lian's ultimate fate was not the same in every version of the game. In the Xbox, PS2, NGC, and Wii versions, it was implied she was crushed to death when her ultimate creation, a "mechanical avatar", was beaten and toppled over. In the PSP and DS versions, Lian collapsed in disbelief at the defeat of her ultimate creation, and Yuan vowed that he and Lian would atone for their actions. The GBA version gave no indication of what happened to Lian and she was not present 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".