Arnook was the Chief of the Northern Water Tribe. Born and raised in the North Pole, Arnook was raised as a warrior, Arnook assumed the role of chief after his father's death during the Third Great Shinobi War. Despite being a nonbender, Arnook was considered a powerful leader, and was the father to Princess Yue and younger brother of desertor Noatak, later known as Amon.
Arnook was born in the Land of Ice as a member of the Northern Water Tribe in 79 AG to Yakone, a fugitive from Republic City, in the Land of Fire, and his wife, a young woman named Innoko, becoming Noatak's younger brother by three years. Together, the family lived peacefully until Yakone discovered Noatak was a waterbender, leading Yakone to brutally train him in the secret ways of bloodbending. Much to Yakone's frustration, Arnook was not a bender, and thus was the subject of his father's physical and emotional abuse.
At age seven, Arnook accompanied Noatak with their father on a supposed hunting trip, far away from their home, where Yakone revealed himself as a crime lord in the Land of Fire and the bearer of a powerful bloodbending kekkei genkai. From then on, Yakone taught bloodbending lessons to Noatak, as Arnook accompanied them, during every full moon, intending to mold his oldest child into a powerful bloodbender to match Konohagakure's shinobi.
After four years of harsh and brutal training which consisted on bloodbending wild animals during a full moon, Noatak learned how to use the forbidden technique without the need of a full moon. He was forced to train constantly, much to Arnook's worry towards his brother, who could not bear seeing Noatak manipulating helpless animals against their will. Intending on hardening Noatak, he ordered his oldest brother to bloodbend Arnook.
Noatak prompted to obey his father without hesitation, subduing Arnook into a painful state of submission. As Yakone went to attack Arnook, however, Noatak subdued him as well to stop him. Noatak then attempted to persuade Arnook into running away with him to start a new life, away from Yakone and his psychothic plans, but the younger brother refused, unwilling to abandon their mother. Noatak then called Arnook a coward, before running away into the blizzard. Arnook called his brother to no avail, and he would search his brother over the tundra for days before giving up, assuming Noatak had perished into the storm.
After Yakone's death in 94 AG, Arnook became the chief of his tribe. He later married the daughter of another tribal chieftain from the Land of Ice. In 110 AG, they had a daughter together, but she became gravelly ill. Desperate to save her life, Arnook prayed to the Moon Spirit. He took his daughter to the Spirit Oasis, where the mortal forms of Tui, the Moon Spirit, and La, the Ocean Spirit, dwelt together. He placed his baby daughter in the water under the full moon, leading her hair to turn white as she was revitalized by the spirit's own life force. In honor of Tui's actions, Arnook's wife named their daughter Yue, for the moon.
During Yue's birth, Arnook had a spiritual vision about her. In this vision, he saw that when she grew mature, she would become the reincarnation of the Moon Spirit.
Arrival of Team Kakashi
When Team Kakashi arrived at the Northern Water Tribe, Arnook was happy to have Naruto Uzumaki, the Avatar, as an ally. He introduced him to Master Pakku, a waterbending teacher. Sakura Haruno, also a waterbender, hoped to learn from Pakku as well, but the master refused duo to the customs of women not using combative waterbending in the Northern Water Tribe. Sakura then begged for Kakashi Hatake and Arnook to have Pakku teach Naruto once again, to which Arnook told it was only Pakku's decision of whom to teach. However, Pakku later agreed to teach Naruto and Sakura waterbending after finding out Sakura had been personally trained by Tsunade, the Fifth Hokage and one of the three legendary Sannin.
When the Land of Fire's military attacked the threshold of the Northern Water Tribe, Arnook called for volunteers for a dangerous mission; Sasuke Uchiha was the first to volunteer himself. After briefing everyone on the mission, Arnook entrusted Sasuke to share the knowledge of the mission to Hahn, the mission's leader and Yue's fiancé. After the two young men fought over "Yue's perks", Arnook ultimately appointed Sasuke as Yue's bodyguard. Despite Sasuke protecting her, he was unable to prevent Yue's fatal destiny during the siege. She then turned into the new Moon Spirit after the previous was killed by Admiral Zhao. Later, Arnook spoke with Sasuke and revealed about his vision when Yue was born.
Arnook was a gentle, wise, and passionate leader who cared about the well being of his people. He saw himself as the wall between them and any threat to the South Pole. A spiritual man, he deeply believed in the power of the Moon and Ocean Spirits, contrary to the perversion preached by his father, Yakone. He pleaded with them to have his ill daughter Yue saved.