Yangchen was the Air NomadAvatar succeeding the unnamed fire Avatar and preceding Avatar Kuruk. In her youth, after mastering all the elements, she was eager to make a difference in the world, despite her inexperience. She traveled with her sky bison and her two winged lemurs, Pik and Pak. A respected figure, she was hailed for her wisdom, determination, and power. Although the Air Nomads were perceived to be pacifistic people, Yangchen was feared for her fierce willingness in doing whatever was required to perform her duties as the Avatar and maintain balance and peace throughout the world. Due to her devotion, no threats of war occurred for an entire generation after her death.
Yangchen was born and raised by Air Nuns in the Western Air Temple. Like all Avatars, she mastered the elements and the Avatar State in order to maintain world peace.
A week after Yangchen had mastered the four elements, she set out with Boma toward a city on a sea shore. Upon her arrival, she was told by its king that the local fortunetellers had predicted a tragedy coming from the sea to befall the city that evening. Believing the fortune, Yangchen waited on a cliff overlooking the water. When past midnight an enormous spirit emerged from the water, she felt an overwhelming grief emanating from the spirit and tried to make contact with it. She was knocked down by it, however, and the two subsequently engaged in battle, which would last through the night and leave the city in ruins. As dawn broke, Yangchen finally managed to convince the spirit to tell her who he was and what he wanted. She could not let him exact his revenge on the citizens for killing Lady Tienhai, however, so she struck a deal with the General Old Iron spirit, one she promised to maintain for eternity in exchange for a truce with the humans. Old Iron promised to never take up his armor against humanity, so long as the land remained free of human civilization. In return, Yangchen constructed a statue in Tienhai's memory and established a ritual among her people to visit the site once a year to prove that humankind could preserve and protect nature. So long as the land remained sacred to the Air Nomads, no one would dare develop the area and risk Old Iron's wrath.
Over time, two statues were made of her: one in the Eastern Air Temple and one in the Western Air Temple. She died around 345 BG, and no threats of war occurred for an entire generation after this date. The Air Nomads continued to take yearly pilgrimages to the statue of Lady Tienhai until they were wiped out, renaming the ritual Yangchen's Festival.
Appearances during Aang's life
Aang first saw Yangchen in in a spiritual vision directed by Avatar Roku, where she was seen generating a strong gust of wind in a large field outside a forest.
Later, Yangchen was sought out by Aang in the Spirit World, at which time the incumbent Avatar was in a coma. During their brief meeting, she discussed Aang's failure at protecting Ba Sing Se from Azula. Amid the young Avatar's embarrassment, Yangchen convinced him to not feel guilty for his shortcoming, as previous Avatars had also made mistakes and struggled to successfully perform their duties. Although she told Aang little about her personal experiences, she was empathetic toward his shame and sadness.
Yangchen also imparted to Aang the reason the Avatar takes human rebirth each time: in order to have a desire to protect humanity, the Avatar must feel compassion toward humans and the only way of doing so was living among them. If the Avatar was instead an all-powerful immortal spirit living at the top of a mountain, he or she would not have anything in common with ordinary people. She acknowledged that the Avatar must experience human life and emotions, such as sadness, anger, joy, and happiness, in order to learn how precious human life is and therefore understand the need to protect it. Thus, with every reincarnation, the spirit of the Avatar comes closer to fully comprehending what it truly means to be human.
At the end of the meeting, Yangchen notified Aang that the spirit of the Avatar had been healed, though his ability to use the Avatar State was disrupted and he would therefore need to rely on his friends, not past lives, for guidance. As Aang departed, Yangchen told the young Avatar that the memory of his journey in the Spirit World would be erased once he awoke from his coma.
When on the lion turtle, Yangchen was the last Avatar Aang summoned for advice on how to defeat Ozai without killing him, thinking a fellow Air Nomad would easily sympathize with his views and support a bloodless victory over the Fire Lord. Although Yangchen remarked the Air Nomad belief that all life was sacred, she reminded Aang that the Avatar should disregard such teachings as they stand in the way of successfully accomplishing the task at hand, which was to defeat Ozai. She told Aang to go against his pacifist beliefs and kill the Fire Lord, which deeply discouraged the young Avatar.
In 102 AG, the day before Yangchen's Festival used to be celebrated, Yangchen appeared as a mysterious hooded figure to Aang in Yu Dao and led him onto a balcony. She tried to speak to him, though Aang could not hear her voice. After she disappeared, he eventually concluded that she came to remind him to revive the celebration. When Team Avatar subsequently made their way into a town that was built upon a meadow sacred to the Air Nomads, she appeared to Aang once more, leading him to investigate the grounds of the Earthen Fire Refinery and discover a polluted river. Some time later, she appeared a third time and imparted a vision upon him of a monster destroying a settlement, leaving him to wonder whether it was a vision of the past or the future.
Later that day, when Aang was saying a blessing before having a ceremonial meal with the Air Acolytes, Yangchen was able to contact him again as he crossed over to the Spirit World. She explained to him that all the Avatars are connected with one another like a chain, so when he broke off his relationship with Roku, he also damaged his connection to all the Avatars that preceded him. However, the ceremonial meal served as a temporary conduit for them to connect. Yangchen told him of her past dealings with General Old Iron, though before she could complete her story and tell Aang the specifics of the deal she struck with the spirit, their connection was interrupted by another earthquake.
Aang later reconnected with his past lives by recrafting the fire medallion on his meditation beads. After reconciling with Roku, Aang made his way down to Yangchen, whom he urged to finish her story. After she did so, the old Avatar was informed that Old Iron's armor had been unearthed, to which she warned Aang that his return was pending. As Yangchen lamented that her promise had been broken, Aang suggested that he could bury the armor once and more and return the land to its natural state. She did not answer, instead telling Aang to return to his friends.
Appearances during Korra's life
Yangchen was among all the other Avatars when Aang passed on the knowledge of energybending to Korra and restored her bending, representing the newly established connection she made to her spiritual self.
While Unalaq was attacking Raava, Korra envisioned Yangchen among the Avatar lineup she had seen prior to regaining her bending; however, her connection to Korra was severed due to Unalaq's assault on and subsequent destruction of the light spirit.
Yangchen, although raised by the Air Nomads, put pacifism aside to fulfill her duties as Avatar. She was eager to perform her duty as the Avatar. Although the Air Nomads teach separation from the rest of the world, she believed that the Avatar can never separate itself from the world because their duty is to it.
When abruptly asked for the name of the female airbender Avatar at the Pacific Media Expo, Bryan Konietzko jokingly dubbed her "Avatar Susan", indicating that they initially did not plan on developing the previous Avatars aside from the two already introduced.