Chief Arnook was the leader of the Northern Water Tribe during the final stages of the Hundred Year War. He was born and raised in the North Pole, where he trained as a warrior. Following his father's death more than twenty years prior to Team Avatar's arrival at the North Pole, Arnook assumed the role of the chief. Though he was not a waterbender, the other men respected him, and he grew to become a great leader. Chief Arnook married the daughter of a tribal chieftain and fathered a daughter, PrincessYue. He was a noble, brave leader who always put the concerns of his citizens first.
In his youth, Arnook trained to be a warrior, and he later became the chief of his tribe after his father passed away around 80 ASC. He married the daughter of a tribal chieftain, and in 84 ASC, they had a daughter together. However, their daughter was gravely ill, and desperate to save her life, Arnook prayed to the Moon Spirit. He then took his infant daughter to the Spirit Oasis, where the mortal form of Tui, the Moon Spirit, dwelt along with the Ocean Spirit. 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; he learned that when she grew to maturity, she would become the reincarnation of the Moon Spirit.
When Aang and the rest of Team Avatar arrived at the Northern Water Tribe, Arnook was happy to have the Avatar present as an ally. He introduced him to Master Pakku, his waterbending teacher. Katara hoped to learn from Pakku as well, but he refused, as it was not customary for women to be taught combative waterbending in the Northern Water Tribe. Later, Pakku refused to teach Aang after catching him teaching Katara waterbending. Katara begged for Arnook to have Pakku teach Aang again, to which Arnook said it Pakku's decision alone whom to teach. However, this problem was resolved when Pakku agreed to teach Aang and Katara after a duel with the latter.
When the Fire Nation attacked the threshold of the Northern Water Tribe, Arnook asked for volunteers for a dangerous mission; Sokka was the first to rise to the occasion. After briefing everyone on the mission at hand, Arnook entrusted Sokka, who had experience fighting the Fire Nation, to share his knowledge with Hahn, the leader of the mission and Yue's fiancé. After the two men got into a fight over "Yue's perks", Arnook took Sokka off the mission and appointed him as Yue's bodyguard instead. Even though Sokka carried out his bodyguard duties, he was unable to prevent Yue's fatal fulfillment of her destiny during the Fire Nation siege. She became the new Moon Spirit after the previous was slain by AdmiralZhao. Later, Arnook spoke with Sokka and revealed the vision he had when Yue was born: a beautiful, brave, young woman would one day become the Moon Spirit.
Arnook was a gentle, wise, and passionate leader who cared deeply about the welfare of his people. He was a very spiritual man who deeply believed in the power of the Moon and Ocean Spirits, pleading with them to save his ill daughter and calling upon them to aid the Northern Water Tribe after realizing the Fire Nation would soon lay siege to the tribe.