Kurama

The is the most powerful out of all nine of the tailed beasts; a single swipe from just one of its nine tails can raise tsunamis and flatten mountains. After the Nine-Tails attacked Konohagakure, Minato Namikaze, the Fourth Hokage, sacrificed his own life by sealing it within his newborn son, Naruto Uzumaki.

Background
The Nine-Tails first came into being in the waning days of the Sage of the Six Paths' life; to prevent its resurgence, the Sage split up the chakra of the Ten-Tailed Beast 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.

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 the 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.

Personality
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.

Land of Waves arc
Naruto was not made aware of the Nine-Tails' presence within him until twelve years after its sealing. He first made use of its chakra during his battle with Haku. Believing Haku to have killed Sasuke, Naruto gave into his anger and entered his initial jinchūriki form in order to defeat Haku.

Chūnin Exam arc
He did not realize that this was the Nine-Tails' chakra until he was later taught to take advantage of it by Jiraiya. To force Naruto to call upon the Nine-Tails' chakra, thus enabling him to use the Summoning Technique, Jiraiya threw Naruto off of a cliff. To save himself, Naruto entered his subconscious and met with the Nine-Tails for the first time. He demanded some of its chakra as "rent" in order to save them both, and the intrigued Nine-Tails complied, allowing Naruto to summon Gamabunta.

Naruto took advantage of the Nine-Tails' chakra again during his fights with Neji Hyūga and Gaara, requesting some of its chakra to help him with the battles.

Search for Tsunade arc
Later, when Kabuto Yakushi severed Naruto's heart muscles and the neighboring chakra system, the Nine-Tails was unable to heal the damage. It, like Naruto, had a near death experience, something it was slow to recognize. Fortunately, Naruto and, through him, the Nine-Tails, were saved by Tsunade.

Sasuke Retrieval arc
During his battle with Sasuke at the Valley of the End, the Nine-Tails gave Naruto enough chakra to attain the one-tailed transformation, something the Nine-Tails indicated that he should be grateful for.

Rescue Gaara arc
During the timeskip, Jiraiya weakened the Nine-Tails' seal with the key in order to give Naruto more access to its chakra. This had the side effect of allowing Naruto to achieve tailed transformations with little provocation. During his battle against the Akatsuki member Deidara, which turned out to be one of the latter's Clay Clones, Naruto slipped him into his two-tailed transformation out of anger at Gaara's death. When he was about to attack Kakashi Hatake, the latter placed a special seal tag given to him by Jiraiya on Naruto's forehead, restoring him back to normal.

Sasuke and Sai arc
Later, when the three-tailed transformation was proven insufficient in a fight with Orochimaru, Naruto gave into the Nine-Tails and entered his four-tailed transformation. The Nine-Tails gained control of his senses and began to attack all around him, including both Orochimaru and Sakura Haruno. After its influence was suppressed by Yamato, he told Naruto how the four-tailed form had caused him to attack Sakura, someone that he cares about. Naruto resolved not to use the Nine-Tails' power again.

Soon afterwards, when Naruto met Sasuke again for the first time in years, the Nine-Tails tried to offer Naruto some of its strength. He refused, but before it could put up much of a fight, Sasuke appeared within Naruto's subconscious. Surprised, the Nine-Tails commented that Sasuke's chakra is more ominous than its own, and that he is just like Madara Uchiha. Sasuke forced the Nine-Tails back into its cage, but before it was completely suppressed, the Nine-Tails warned Sasuke not to kill Naruto, telling him that he would only live to regret it. It saw little activity after that, surfacing within some of Naruto's shadow clones that had grown frustrated with the wind element training.

Twelve Guardian Ninja arc
In the anime, the Nine-Tails reacted to its leftover chakra that was stored within Sora. As Naruto in his three-tailed form battled Sora in his four-tailed form, the Nine-Tails started to force Naruto to accept its power, but he was able to keep the Nine-Tails from taking control of his body as he stabbed his leg and threatened the beast to back off. It begrudgingly pulled back, but it managed to take back all of its chakra from Sora.

Invasion of Pain arc
Later, when Naruto tried to learn senjutsu, the Nine-Tails prevented Fukasaku from fusing with Naruto, wanting Naruto to use its chakra instead of senjutsu chakra. It was given an opportunity to appear during Naruto's battle with Pain. After Pain appeared to have killed Hinata Hyūga, Naruto slipped into his six-tailed transformation, which the Nine-Tails used to its advantage by destroying the First Hokage's Necklace.

When Naruto was captured by Pain's Chibaku Tensei technique, the Nine-Tails tempted Naruto to give his heart to it so that it could destroy everything that was causing Naruto pain, thus distorting the seal to the point where the eight-tailed transformation was released. Before it could convince him to completely remove the seal, however, the spirit of Minato appeared in Naruto's subconscious to stop him. Enraged, the Nine-Tails tried its best to kill Minato for revenge, but Minato ignored it, soon afterwards restoring the seal to its original strength. Once Pain was defeated, Naruto confronted Pain's base mind, Nagato, and was able to simultaneously use senjutsu chakra and the Nine-Tails' chakra, combining the two different pupil appearances.

Activity inside Naruto
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
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, 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.

Influences
A is a well-known character in Japanese folklore, and is the influence behind not only the Nine-Tails, but many other reincarnations in modern Japanese culture. It likes to play pranks on people, and, on occasions, transforms itself into a beautiful woman in order to seduce men (obviously the inspiration for the Sexy Technique). However, kitsune are known to keep their promises, a trait Naruto has; his ninja way is to "never go back on [his] word". It is also noted that, every time Naruto loses control and is overwhelmed by the Nine-Tails' chakra, he gains another 'tail' of chakra. This stems from the belief that the more tails a kitsune has, the more powerful it is, with nine tails being the limit.

Trivia

 * According to the third Naruto Databook, the Nine-Tails is the only one who knows the truth about the Uchiha clan and about Naruto. However, it appears that it has no intention of telling what it knows.
 * Based on its reaction, Naruto almost dying at the hands of Kabuto was the only time the Nine-Tails was close to death or to that matter, suffered any injury it could not simply heal through.

Quotes

 * (To Naruto) "(Laughs) Well, I suppose I can't kill you, can I? After all, you die, I die. Your courage is most impressive, little boy... Very well, as a reward for making it this far, I will loan you my chakra!"
 * (To Naruto) "You runt, you really are weak! (Laughs) You better be grateful to me... And to your beloved Fourth Hokage, who sealed me inside a twerp like you!"
 * (To Sasuke) "With that vision of yours, and chakra more ominous than my own... You're the spitting image of Madara Uchiha from the elder days."
 * (To Naruto) "Destroy everything... Destroy everything that hurts you... Give your heart to me... I will save you from the pain."