The Nine-Tails (九尾, Kyūbi) is a tailed beast sealed within Naruto Uzumaki.
The Nine-Tails is a large kitsune with red-orange fur, red eyes, and the upper-body structure of a human, complete with opposable thumbs on its clawed hands.
The Nine-Tails is a cynical and shrewd individual, with a distinct sense of pride. It uses 'washi' (ワシ) when referring to itself, which is generally used by older men. The Nine-Tails resents its imprisonment inside of Naruto and has tried to find any kind of opprotunity to break the seal.
The Nine-Tails is first seen when the team was having trouble fighting against a group of Takigakure shinobi (due to Naruto and Lee switching bodies), it used the opportunity to inveagle "Naruto" into accepting its power, but was surprised when Lee appeared, who referred to it with the honorific "kun" and offered a green bodysuit as a present, to which the Nine-Tails instantly rejected it, saying it had no use for it. When Lee brought out a bone for it as a replacement, the fox felt insulted and started to yell at him. Seeing this, Lee came up with a parody of the Japanese movie "Nankyoko Monogatari", portraying the Nine-Tails as Taro (a small puppy-like version of the tailed beast), as his last attempt to try to befriend the Nine-Tails, but the irritated fox just simply scoffed it off. Noticing that Naruto and Lee's switched body situation had weakened the seal, the Nine-Tails made "Naruto" transform into his two-tailed chakra shroud (but chakra manifested themselves on his eyebrows instead of as tails shocking everyone). The Nine-Tails' chakra was soon suppressed by Kakashi with a Sealing Tag.
The Nine-Tails has a massive insurmountable amount of chakra that it lends to Naruto ocassionally. This chakra is red in color and covers up its host/Naruto's whole body.