Kurama

The is the most powerful of the tailed beasts; a single swipe of one of its nine tails can raise tsunamis and flatten mountains. After the demon fox attacked Konohagakure, Minato Namikaze, the Fourth Hokage, sealed it within his newborn son, Naruto Uzumaki, by using the Dead Demon Consuming Seal, sacrificing his own life to do so.

Background
The demon fox was an age-old natural disaster, appearing at random to attack areas where human malice collected and festered. Many years ago, when Madara Uchiha defected from Konoha, he used his Mangekyo Sharingan to take control of the demon fox, 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. However, despite being in control of the demon fox's power, Madara was defeated.

Many years later, Madara used the demon fox again to attack Konoha (although Madara denied this, and claimed that the demon fox's attack had been a "natural occurrence"). During the attack, the demon fox decimated all of the shinobi that tried to repel it. Minato, riding atop Gamabunta, was able to defeat the demon fox by sealing it within Naruto, which freed it from Madara's control. In the anime, it was revealed that the demon fox had left behind large amounts of its chakra during the attack. Kazuma, hoping to make use of its power, sealed the chakra within his son, Sora.

Personality
The demon fox is a giant beast of mass destruction and almost pure malevolence. 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 demon fox 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 demon fox 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've weakening the seal. However, it will back down if Naruto threatens to harm or kill himself.

The demon fox 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's eyes and chakra are more sinister than its own, as his Sharingan reminded it of Madara. According to the Third Naruto Databook, the demon fox is the only one who knows the truth about the Uchiha clan and about Naruto. However, it appears that the demon fox has no intention of telling what it knows.

Part I
Naruto was not made aware of the demon fox's 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. He didn't realize that this was the demon fox's chakra until he was later taught to take advantage of it by Jiraiya. To force Naruto to call upon the demon fox's 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 demon fox for the first time. He demanded some of its chakra as "rent" in order to save them both, and the intrigued demon fox complied, allowing Naruto to summon Gamabunta.

Naruto took advantage of the demon fox again during his fights with Neji Hyuga and Gaara, requesting some of its chakra to help him with the battles. Later, when Kabuto Yakushi severed Naruto's heart muscles and the neighboring chakra system, the demon fox was unable to heal the damage. It, like Naruto, had a near death experience, something it was slow to recognize. Naruto, and, through him, the demon fox, were saved by Tsunade. During his battle with Sasuke at the Valley of the End, the demon fox gave Naruto enough chakra to attain the one-tailed transformation, something the demon fox indicated that he should be grateful for.

Part II
During the timeskip, Jiraiya weakened the demon fox's 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. When the three-tailed transformation was proven insufficient in a fight with Orochimaru, Naruto gave into the demon fox and entered his four-tailed transformation. The demon fox 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 demon fox's power again.

Soon afterwards, when Naruto met Sasuke again for the first time in years, the demon fox 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 demon fox commented that Sasuke's eyes and chakra are more evil than its own, and that he is just like Madara Uchiha. Sasuke forced the demon fox back into its cage, with the demon fox warning Sasuke not to kill Naruto before it disappeared. It saw little activity after that, surfacing within some of Naruto's shadow clones that had grown frustrated with the wind element training. In the anime, the demon fox reacted to its chakra stored within Sora. Naruto was able to keep the demon fox from taking control of his body, but it managed to take back all of its chakra from Sora.

Later, when Naruto tried to learn senjutsu, the demon fox prevented Fukasaku from fusing with Naruto, wanting Naruto to use its chakra instead of Sage chakra. It was given an opportunity to appear during Naruto's battle with Pain. After Pain appeared to have killed Hinata Hyuga, Naruto slipped into his six-tailed transformation, which the demon fox used to its advantage by destroying the First Hokage's Necklace. When Naruto was captured by Pain's Chibaku Tensei technique, the demon fox 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 demon fox 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 Sage chakra and the demon fox's chakra, combining the two different pupil appearances.

Activity inside Naruto
As the demon fox's jinchūriki, Naruto has accelerated healing, unrivalled stamina and speed, massive strength, and access to the demon fox's near limitless chakra. The demon fox's 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 demon fox's chakra naturally mixes with Naruto's own, Naruto can forcefully tap into the demon fox's reserves. After training with Jiraiya, he learned to request some of the demon fox's 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 demon fox 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. For this reason, Naruto learned to exercise more control over the demon fox's chakra.

A number of methods have been devised to break the demon fox's influence over Naruto: Yamato can use the Hokage-style Sixty-year-old Technique - Kakuan Entering Society with Bliss-bringing Hands, which is facilitated by the necklace Naruto wears; a Demon Fox Chakra Seal can be applied to Naruto's body; or a Uchiha can enter Naruto's subconscious and force the demon fox to recede. The most effective of measures, however, is Naruto simply limiting his use of the demon fox's chakra and not allowing his anger to get the better of him.

The Seal
This is the seal that imprisons the demon fox within Naruto. The Dead Demon Consuming Seal is composed of the Eight Trigrams Sealing Style, which allows Naruto to tap into the demon fox's chakra reserves in times of need. Jiraiya later revealed that, when Minato had sealed the demon fox within Naruto, he had split the demon fox's chakra into two halves, the Yin and Yang. He sealed the Yang part within Naruto with the Eight Trigrams Sealing Style, and sealed the Yin part with the Dead Demon Consuming Seal, thus making it inaccessible to Naruto.

Whenever Naruto and the demon fox converse, it occurs within Naruto's subconscious, which is depicted as a kind of 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 demon fox's 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 demon fox's cage, there is a parchment of paper that says, which serves as a 'lock' to keep the demon fox in.

Hiruzen Sarutobi and Kakashi Hatake alluded that, if Naruto loses all control of the demon fox's chakra, the seal will break and the demon fox 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 demon fox's 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 demon fox's 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 demon fox's 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 demon fox 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 demon fox, 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, kitsunes 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 demon fox's 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 usually being the limit.

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) "Eyes with such power and chakra more sinister than my own ... you're just like Madara Uchiha."
 * (To Naruto) "Destroy everything ... destroy everything that hurts you ... give your heart to me ... I will save you from the pain."