Yue was born with an unknown illness; as an infant, she was still and quiet, as though asleep, and her parents feared she would die. As her condition was critical and the tribe's healers - including Yugoda - could not cure her, Arnook prayed to the Moon Spirit in desperation and placed her in the tribe's Spirit Oasis. The Moon Spirit gifted her with part of its own life force, saving her life and turning her hair white as a side effect. Thus, her parents named her Yue, for the moon. Sometime prior to Team Avatar's arrival at the Northern Water Tribe, it was arranged that Yue would marry a young, egotistical warrior named Hahn who carved a betrothal necklace for her to mark their engagement. It was later revealed, however, that she did not love him.
Upon arriving at the Northern Water Tribe, Sokka was smitten with her, taken by her beauty at first sight. After awkwardly flirting with her, the two soon began to arrange meetings. These included riding on Appa and taking walks together. Although affection budded between the two, Yue was bound in an arranged marriage to Hahn, who had more love for her high status than for Yue herself. He boasted once to Sokka that "she [came] with the most perks". Sokka and Yue came to realize their love for each other, but she could not bear to continue their meetings because they conflicted so strongly with her duty to her tribe. Sokka eventually became angered when Yue insisted that being together was "wrong" and that it was her duty to marry Hahn for her people, to which Sokka replied she was not marrying her people. Both of their feelings persisted, and during AdmiralZhao's siege of the Northern Water Tribe, Sokka was ordered by Arnook to act as Yue's bodyguard.
Yue aided Aang in entering the Spirit World, by leading him and Katara to the Spirit Oasis. Later, when Aang had been captured by Zuko, she accompanied Katara and Sokka as they tracked the two of them down on Appa. Upon their return to the Spirit Oasis, they were confronted by Zhao, who fatally struck the Moon Spirit's mortal form, leaving all waterbenders powerless without the moon's influence. Iroh, who appeared on scene, saw that she had been touched by the Moon Spirit. He reminded Yue of this, and she once again acted dutifully to her tribe, sacrificing herself in order to give back the life that the Moon Spirit had given her as a newborn. Her sacrifice was made against Sokka's wishes, but in the end, the young warrior did not stop her. She placed her hands onto the fish, closed her eyes, and died in Sokka's arms.
Her spirit appeared before him shortly after, wearing a long white gown. She whispered, "Goodbye, Sokka. I will always be with you," and the two shared a final kiss before she vanished completely.
Several times following her death, Yue's influence on Sokka returned. While in the Foggy Swamp, Sokka, under the swamp's magical influence, saw a vision of her. The vision of Yue angrily told Sokka that he failed to protect her, perhaps a manifestation of his fear that she truly felt this way. During his time in the Si Wong Desert, Sokka directly referred to the moon as Yue in the intoxicated state brought about by drinking cactus juice.
When Sokka and Suki nearly kissed while crossing the Serpent's Pass, a waning gibbous moon is visible in the background, alluding to Yue's presence, and Sokka's memories of Yue's death prompted him to refuse the kiss, while acting overly protective of Suki. While looking for leads as to disappearances surrounding the full moon, Sokka yelled at Toph when she suggested that the moon spirit might be harming people, saying that the moon spirit "is a gentle, loving lady who rules the skies with compassion and lunar goodness", showing that he still cared deeply for Yue. While with Zuko on the way to the Boiling Rock, Sokka mentioned that "[his] first girlfriend turned into the moon."
After Aang was struck down by Azula's lightning during the Coup of Ba Sing Se, the Avatar met Yue in the Spirit World. She informed him of the damage that had taken place to the Avatar Spirit and explained how to return to his body, further instructing him to seek out the past four Avatars while avoiding Koh, the Face Stealer. She later appeared to Aang once more to encourage him not to lose hope and to continue fighting. She then assisted him in his impressive feat of waterbending to get himself to safety, utilizing her powers as the Moon Spirit.
During a play that parodied the Siege of the North, Yue was played by someone who, as the play's version of Sokka said, is the "only woman who's ever taken his mind off food", before she floated away on a crescent moon prop. In the audience, Sokka was crying as Suki told him he never told her that he had kissed the Moon Spirit. Sokka shushed her, still crying, showing that he still cared for Yue.
Yue was kind, sweet and gentle. She had a deep sense of loyalty and duty to her tribe. She offered hope to others, and was courageous enough to sacrifice her own life to save the Moon Spirit and the Water Tribe. Along with her compassion, however, Yue also hid a great deal of guilt over being openly affectionate toward Sokka while engaged to Hahn. She was highly spiritual, referencing the Moon and Ocean Spirits several times; this was probably due to her connection with Tui. It was also hinted that she had a serious side, as she glared at Momo when he tried to catch a sacred koi in the Spirit Oasis.
During the time in which Team Avatar visited the North Pole, Yue did not seem to possess any waterbending ability - not even healing abilities, the only aspect of waterbending taught to women in the Northern Water Tribe - despite being a spiritual person. After offering her life to resurrect the Moon Spirit, she became a waterbender as well. Just after renewing Aang's faith, she helped him to safety by enlarging the wave he had created to carry him to shore.
"Yue" (月) means "moon" in Mandarin Chinese. In Japanese, it is pronounced as "you-eh", as heard in the series.
The name also means "tragic accident" in Japanese (夕映) and is traditionally given to Japanese children who die shortly after birth.
Yue's appearance as the Moon Spirit is very similar to that of (嫦娥), the Goddess of the Moon in Chinese mythology.
Yue and Kanna were the only two polar Water Tribe members not to wear any blue clothes. They wore purple instead.
When Yue became the Moon Spirit, her betrothal necklace vanished.
When Yue showed Sokka her betrothal necklace, it was the only time it was seen throughout the series.
Yue was the only character who, when introduced, did not seemingly have the ability to bend, but then was able to because of her taking on the role as the Moon Spirit.
As the Moon Spirit, Yue's hair was sometimes down along with her hair-do on top.
During Yue's death, the soundtrack plays part of a tune with great similarity to Iroh's "Four Seasons" song, which was about the four nations all existing together and about all the elements, likely in reference to Yue's decision to preserve the necessary balance of the world no matter the cost. This song is also present during the scenes when she is with Sokka.
It was seen that after the Moon Spirit was killed, everything turned gray, but Yue's eyes remained blue, showing the life force of Tui inside of her.
Yue was the first of two characters, Aang being the second, but revived later, whose death appears on-screen.