|Nine-Tailed Demon Fox|
|Current Status||not captured|
|First Appearance|| Anime: Episode 1|
Manga: Chapter 1
The Nine-Tailed Demon Fox (Kyūbi no Yōko), or Isobu (The name given by the Sage of Six Paths), simply referred to as the Kyuubi in the Japanese anime and fans of the series, is the ninth and most powerful of all the tailed beasts (bijū), which are large manifestations of unlimited chakra. The Nine-tailed Fox, as the name implies, has nine tails, which according to legend signifies that the fox has attained immortality. It is said that one swoop of just one of its tails is able to create tsunamis and destroy mountains. The fox, after nearly causing the complete annihilation of Konoha, was sealed into infant Naruto by the Fourth Hokage. Although the seal the Fourth Hokage placed on Naruto prevents the nine-tails from escaping, it was designed in a fashion that the fox's chakra could still leak out and therefore provide Naruto with the fox's immense power.
 Appearances and History
CONTAIN SPOILERS The Nine-Tailed Fox is one of the Nine Bijuus. Each bijuu, or Tailed Beasts, each have a corresponding number of tails counting from 1-9. In the past the Nine Bijuus used to be just one being called the Ten-Tailed Beast. The Ten-Tailed Beast roamed and destroyed the land, so the Sage of the Six Paths absorbed it into himself making himself the first jinchuriki, when his death came closer, he decided to separate the Ten-Tails to separate its enormous powers thus creating the Nine Bijuus.
The nine-tailed fox made its first appearance twelve years prior to Naruto's graduation from the ninja academy. In an unforeseen catastrophe, the Nine-tails appeared out of nowhere and laid waste to almost all of Konoha, killing a large number of ninja in the process. However, the fox's rampage came to an abrupt end when the Fourth Hokage sealed the fox into infant Naruto. The Nine-tails throughout the Naruto series has made brief appearances, however inside Naruto's subconscious, usually arguing with Naruto about lending its strength to Naruto whenever he's in life threatening situations. As the story progresses, evidence has shown that the seal that holds the Nine-tails inside Naruto is quickly weakening, making it easier for the demon-fox's chakra to seep out and enshroud Naruto. Nevertheless, Naruto thus far has been able to suppress the fox's chakra mostly by his own willpower (and thanks to the First Hokage's necklace which he wears around his neck), though other times he requires Yamato's assistance if he's around.
Later on in the storyline, in a history briefing given by Madara Uchiha to Sasuke Uchiha, the Nine-tailed Fox is revealed to have fought alongside Madara, however under Madara's control due to the power of the Mangekyō Sharingan, against the First Hokage.