The Nine-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent the Ten-Tailed Beast resurgence, the Sage split up its chakra into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. Out of all the nine tailed beasts, the Nine-Tails is the most powerful and its form is also the closest to the form of the Ten-Tails. Over the centuries, the Nine-Tails gained a reputation as an age-old natural disaster, appearing at random to attack areas where human malice has collected and festered. The battle between Madara and Hashirama.Many decades ago, when Madara Uchiha defected from Konoha, he used his Mangekyō Sharingan to take control of the Nine-Tails, and used it to help him fight against Hashirama Senju, the First Hokage, to exact his revenge. The battle was so great that it tore the earth apart and created the Valley of the End. Despite being in control of the Nine-Tails' power, Madara was defeated.
Many years later, the Nine-Tails attacked Konoha. Madara denied that he had summoned it to destroy the village and claimed that the Nine-Tails' attack had been a natural occurrence. During the attack, the Nine-Tails decimated many shinobi that tried to repel it. Minato, riding atop Gamabunta, was able to defeat the Nine-Tails by sealing it within Naruto.
In the anime, it was revealed that the Nine-Tails had left behind large amounts of its chakra during the attack. Kazuma collected and sealed the chakra within his own son, Sora, hoping to make use of its power for his own agenda.
The Nine-Tails is a giant, malevolent demon fox of mass destruction. It is also intelligent, and has a sadistic and sarcastic personality. However, it does have a distinct sense of honor and pride, and possesses a loathing respect for Naruto and Minato.
At first, the Nine-Tails relished the thought of killing 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. 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 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 or kill 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.
As the Nine-Tails' jinchūriki, Naruto is gifted with massive chakra and stamina, immense strength, increased speed, accelerated healing, and, at times, an almost impenetrable chakra shield. The Nine-Tails' chakra allows Naruto to perform his signature jutsu, Shadow Clone Technique, seemingly without limit. He is even able to summon Gamabunta, a feat that is normally Kage-level as the technique requires an amount of chakra proportionate to the size of the summoned creature.
Although the Nine-Tails' chakra naturally mixes with Naruto's own, Naruto can forcefully tap into its chakra reserves. After training with Jiraiya, he learned to request some of the Nine-Tails' chakra for his own use. Alternatively, and what has become the more common method over time, he can give into his anger or frustration, allowing the Nine-Tails to force its influence upon him. Depending on how much of its chakra he gains, Naruto can lose control of his senses, devolving into an animal that attacks anything within reach, as well as damaging his body with the Nine-Tails' chakra, slowly shortening his lifespan. For these reasons, Naruto learned to exercise more control over the Nine-Tails' chakra.
A number of methods have been devised to break the Nine-Tails' influence over Naruto: Yamato can use the Hokage-style Sixty-year-old Technique - Kakuan Entering Society with Bliss-bringing Hands, which was facilitated by the necklace Naruto wore before it's destruction; a Chakra-Suppressing Seal can be applied to Naruto's body; or an Uchiha with a mature enough Sharingan can enter Naruto's subconscious and force the Nine-Tails to recede.The most effective of measures, however, is Naruto simply limiting his use of the Nine-Tails' chakra and not allowing his anger to get the better of him.
The Seal The seal left by the Dead Demon Consuming Seal.This is the seal that imprisons the Nine-Tails within Naruto. The Dead Demon Consuming Seal is composed of the Eight Trigrams Sealing Style, which allows Naruto to tap into the Nine-Tails' chakra reserves in times of need. Jiraiya later revealed that, when Minato had sealed the Nine-Tails within Naruto, he had split its chakra in two, the Yin and Yang. Minato sealed the Yang half within Naruto with the Eight Trigrams Sealing Style, and sealed the Yin half with the Dead Demon Consuming Seal, thus making it inaccessible to Naruto.
Whenever Naruto and the Nine-Tails converse, it occurs within Naruto's subconscious, which is depicted as a basement of a boiler room or a sub-level of a power plant, with pipes running across the ceiling and the floor being flooded over with water. The water represents the Nine-Tails' chakra that leaks out of the seal and is channelled into Naruto's chakra system; its level indicates how much of its chakra is being drawn out. On the entrance gate of the Nine-Tails' cage, there is a parchment of paper that says "seal" (封), which serves as a 'lock' to keep the Nine-Tails in.
Hiruzen Sarutobi, the Third Hokage, and Kakashi alluded that, if Naruto loses all control of the Nine-Tails' chakra, the seal will break and the Nine-Tails will be released. Although the seal is unlikely to break on its own, the chances of it happening increase as Naruto draws upon more of the Nine-Tails' chakra. The seal naturally weakens over time, something Minato created a key, Gerotora, to remedy. Jiraiya instead used Gerotora to 'twist' the seal a little, which inadvertently caused more of the Nine-Tails' chakra to leak out and unleashed the four-tailed transformation. As the seal weakens, Naruto's transformations occurs much quicker and with greater intensity, often against his own will. As seen during Naruto's Part II training sessions, even the slightest amount of frustration on Naruto's part allows the Nine-Tails' chakra to leak out of the seal, which will then rapidly progress to his four-tailed form, or onwards.
The seal also has a final safeguard to prevent the Nine-Tails from escaping. Minato made it so that, if Naruto ever transformed into his eight-tailed form, Minato would appear in his subconscious in order to prevent the seal from being completely broken. Furthermore, Minato can restore the seal to its normal state, but he stated that he can only do it once.
- The kyuubi's name is Kurama