When Arnook's daughter Yue was first born, she was gravely ill. Desperate to save her life, he prayed to the Moon Spirit. He then took his infant daughter to the Spirit Oasis, where the mortal form of Tui, the Moon Spirit, dwelled. He placed his daughter in the water under the full moon, and her hair turned white, as she was revitalized by a measure of the spirit's own life force. His wife then named her Yue, for the moon.
At Yue's birth, Arnook had a spiritual vision of his daughter, when grown to maturity, becoming the reincarnation of the Moon Spirit.
Arrival of the Avatar and the Fire NationEdit
When Aang and the gang arrived at the Northern Water Tribe, Arnook is happy to have the Avatar present as an ally. He introduces him to Master Pakku as his Waterbending teacher. After Pakku refused to teach Aang after catching him teaching Katara Waterbending, Aang and the gang begged for Arnook to have Pakku teach Aang again. He said he couldn't force him to do it because it was up to Pakku to decide whom he teaches. However, this problem was resolved when Pakku agreed to teach Aang and Katara after a duel with the latter.
When the Fire Nation attempted to invade the Northern Water Tribe, Arnook asked for volunteers to help fight the Fire Nation (Sokka being one of them). Arnook briefs everyone on their mission, but then tells to Sokka do it instead, after finding out that he knew more about the attackers than he did. After Sokka gets into a fight with Yue's fiancé, Hahn, he takes Sokka off the mission. He then appoints Sokka as Yue's bodyguard. Even though Sokka did as Arnook said, his daughter fulfills her destiny during the Fire Nation siege. Later, Arnook speaks with Sokka, and reveals the vision he had when Yue was born that the beautiful princess would one day become the Moon Spirit.