Name : Kyuubi no Youko, "Nine-tailed Demon Fox", Demon Fox (Viz), English TV "The Nine-tailed Fox"
Creature Type : Kitsune (Fox)
Jinchuuriki : Uzumaki Naruto
Signature Ability : Chakra transfer, a shield of red chakra that resembles Gaara's absolute defense.
The Demon Fox is said to be the most powerful of the tailed-beasts; a single swipe of one of its nine tails could raise tsunamis and flatten mountains. After the Kyuubi attacked Konohagakure, the Fourth Hokage sealed it into baby Naruto by using the Shiki Fuuin (Dead Demon Seal), sacrificing his own life.
The fox is a beast of almost pure malevolence. It is intelligent, and has a scathing, sadistic personality. However, it does not lack for a sense of honor, and seems to possess a respect (if merely a loathing one) for Naruto, and for the Fourth Hokage. Additionally, it has a distinct sense of pride and became rather angry and scornful at Naruto when Naruto rejected it and its chakra when facing Sasuke in the Grass Country arc.
The Jinchuuriki Naruto enjoys accelerated healing and access to immense amounts of chakra, which increase all areas of physical strength. The near-limitless allows to perform his signature jutsu, Kage Bunshin no Jutsu (Shadow Doppelganger Technique), to near limitless degrees as well as override the forbidden jutsu's hazard of dividing the user's limited chakra among the clones. Also, he is able to manipulate his chakra for close or long range attacks. He is even able to summon Gamabunta the "Toad Boss", a normally Kage-level feat requiring the amount of chakra proportionate to the size of the summoned creature.
Naruto typically acquires the prodigious supply of chakra from the Kyuubi when in an extreme emotional state and in life-threatening situations. Although after Jiraiya's training, it is evident that Naruto can summon a small degree Kyubi's power without losing his mind, particularly his no-tail to the two-tailed transformed states. The side effect of Naruto going berserk, at three or more tailed transformations, is that the Kyuubi would start taking over his personality, appearing as a fox-shaped aura of red chakra surrounding him. The level of danger is indicated by the number of tails the aura would grow. So far, Naruto has been seen bearing four tails, at which point the Kyuubi personality becomes dominant. In this state, Naruto becomes highly destructive and unable to distinguish between friend and foe, and his body suffers severe damage due to an effect where the aura seems to be providing a shield but in reality is burning him. This repeated process of damaging and healing is said to weaken Naruto and shorten his lifespan because of the constant cell regeneration, much like Tsunade's accelerated regeneration jutsu.
In the event of the Kyuubi manifesting, Naruto's sensei, Jiraiya, prescribes a special written seal placed on his body to suppress the Kyuubi's chakra leakage and restore his sobriety. In addition, Yamato has demonstrated a technique that will reverse Naruto's Kyuubi transformation.
The Third Hokage and Hatake Kakashi allude that if Naruto loses all control of the Kyuubi's chakra, the seal will break and the Kyuubi will be freed. The chances of this happening are one in a million, but increase the more enraged and out-of-control Naruto becomes. In recent chapters, Jiraiya has mentioned that the seal may be becoming weaker.
Recently, it was revealed that Kyuubi knew relatively well about the secrets of Uchiha clan. The precise relationship between the Uchiha clan and Kyuubi is unclear as of now. What is known is that a clan member called Uchiha Madara's chakra is/was more evil ("cursed", as described by Kyuubi) than the Kyuubi's own chakra, and that some Uchiha (including Sasuke) can surpress the chakra of the Kyuubi.