Avatar: The Last Airbender introduced fans to a magical world where people could control the elements, bison could soar through the sky and your girlfriend... could turn into the moon? Princess Yue was a briefly-seen character introduced toward the end of the show's first season, but she played a critical role in the season's finale before going on to aid the cause of global harmony and several crucial junctures. She's easy to overlook, but there's no doubting Yue's importance.
Even from her introduction, it was clear that Princess Yue was an important figure, the teenage daughter of the Northern Water Tribe's Chief Arnook. The moment Sokka laid eyes on her he was smitten, but the deeply-ingrained traditions of the Northern Water Tribe dictated that she was already betrothed to another. Though it seemed she returned Sokka's feelings, she struggled against the traditions of her upbringing and her important role as a political figure.
Fate has a funny way of working, however, and it was not in politics that Yue's destiny lay at all. Nor, indeed, was she destined to be Sokka's romantic partner. Instead, Yue returned a gift owed to her from early childhood. As an infant, she had a deadly illness that threatened her life, and after hearing her parents' prayers the Moon Spirit granted her a piece of its life force. When Admiral Zhao slayed the Moon Spirit during his invasion of the Yue's home, she returned the spirit's gift, restoring the moon in her tribe's most desperate hour of need and joining one of the most powerful forces in the world. But her world-saving days had only started.
Her presence continued to linger over Team Avatar during the show's second season, with Sokka seeing her as a vision in the Foggy Swamp and staying in his thoughts during his hallucinations in the Si Wong Desert and overlooking his reunion with his new girlfriend Suki in the Serpent's Pass. But it was not until the end of the second season that Yue stepped in to take a more active role in the world's harmony.
Following his death from Azula's lightning bolt at the end of the second season, Aang found himself in the Spirit World where Yue greeted him as a friendly face. With Katara using water from the Spirit Oasis to restore Aang to life, Yue informed Aang of the danger he was in and the importance of restoring the Avatar Spirit and returning to his body. When Aang did awake, he overexerted himself while flying out to sea and Yue once again appeared to him and helped waterbend a tidal wave he could ride to safety. And she was just getting started.
A crucial battle in the Hundred Years War was the Day of the Black Sun, when a solar eclipse robbed firebenders of their power and left them highly vulnerable to attack. Anticipating such an event, Team Avatar organized an invasion of the Fire Nation timed perfectly for that event, finding little resistance once the firebenders were unable to produce flames. Simultaneously, King Bumi liberated Omashu after freeing himself from prison, viewing the eclipse as the perfect opportunity he'd been waiting all that time for since his capture. And it was all only possible because the Moon lent a hand.
Little is known if Yue ever appeared to any members of Team Avatar following the Hundred Years War, but as far as the canon goes, it would still be her up there, providing waterbenders their magnificent power while contributing her part to the cycle of nature. It certainly helps to have friends in high places, and it's hard to get friends in much higher places than the moon.