The Nine-Tailed Demon Fox (九尾の妖狐, Kyūbi no Yōko) is the most powerful out of all nine of the tailed beasts; a single swipe from just one of its nine tails can create tsunamis and flatten mountains. When the Nine-Tails attacked Konohagakure, Minato Namikaze, the Fourth Hokage, sacrificed his own life by sealing it within his newborn son, Naruto Uzumaki. It was later passed on to his son, Hiroshi Hyuuga
Although the Nine-Tails primarily resembles a demonic kitsune with orange fur and blood-red slitted eyes, it also possesses the upper-body structure of a human, complete with opposable thumbs on its pawed hands. Out of all the tailed beasts, the Nine-Tails' form is closest to the form of the Ten-Tails.
The Nine-Tails is malevolent, sadistic, cunning, and sarcastic, but it does have a distinct sense of honor and pride, and possesses a loathing respect for Naruto and Minato. At first, the Nine-Tails wanted to kill Naruto, but it soon realized that it shares the same fate as Naruto as long as it is sealed within him; so it has no choice but to cooperate with Naruto or it will die along with him. It will go to great lengths to preserve its own life, as shown when it forces its chakra into Naruto whenever his life is in danger. Later in Part II, the Nine-Tails became very angry and contemptuous at Naruto when he rejected its chakra, although this was merely the rage of missing the opportunity to usurp control over Naruto's mind and body, which would have weakened the seal. However, it will back down if Naruto threatens to harm himself.
The Nine-Tails has a deep hatred for the Uchiha clan and their Sharingan, likely because of the many times Madara manipulated it in the past. It even remarked that Sasuke Uchiha is a lot like Madara because of his power of the Sharingan. The Nine-Tails actually knows the truth about the Uchiha clan and about Naruto, but it appears that it has no intention of telling what it knows.