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 create tsunamis and flatten mountains. When the Nine-Tails attacked Konohagakure, Minato Namikaze, the Fourth Hokage, sacrificed his own life by sealing it within his newborn son, Naruto Uzumaki.

Background
According to Madara Uchiha, 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's resurgence, the Sage split up its chakra into nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. Over the centuries, the Nine-Tails gained a reputation as an age-old natural disaster, appearing suddenly out of nowhere to attack areas wherever human malice has collected and festered.

Many decades ago, when Madara defected from Konoha, he used his Eternal 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 and was believed to have died.

Twelve years before the start of the series, after the end of the Third Great Shinobi World War, the Nine-Tails attacked Konoha. Although Madara denied that he had any involvement and claimed that its attack was a natural occurrence, Jiraiya, Itachi Uchiha, and Minato Namikaze said that the Nine-Tails was under Madara's control at the time. During the attack, the Nine-Tails decimated many shinobi that tried to repel it. Minato, riding atop of 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.

Appearance
Although the Nine-Tails primarily resembles a demonic kitsune with orange fur and blood-red slitted eyes, it also possesses the upper-body structure of a human, complete with opposable thumbs on its pawed hands. Out of all the tailed beasts, the Nine-Tails' form is closest to the form of the Ten-Tails.

Personality
The Nine-Tails is malevolent, sadistic, cunning, and sarcastic, but it does have a distinct sense of honor and pride, and possesses a loathing respect for Naruto. At first, the Nine-Tails wanted to kill 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 along with him. 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 very 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.

Just as the Eight-Tailed Giant Ox commented that it did not like the Nine-Tails, the Nine-Tails has shown indifference towards the Eight-Tails, rebuking Naruto for needing help controlling it with the help of the Eight-Tails. Moreover, the Nine-Tails' reaction to Naruto's new seal indicated that it also had knowledge of the Sage of the Six Paths, and was thus appeared to have knowledge about the Ten-Tails.

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. The Nine-Tails actually knows the truth about the Uchiha clan and about Naruto, but it has no intention of telling what it knows.

Activity inside Naruto
As the Nine-Tails' jinchūriki, Naruto is gifted with its mass amount of 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: a Chakra-Suppressing Seal can be applied to Naruto's body; Yamato can use the Hokage-style Sixty-year-old Technique - Kakuan Entering Society with Bliss-bringing Hands, which was facilitated by the First Hokage's Necklace Naruto wore before its destruction; 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.

According to Gerotora, the Nine-Tails' power consists of two parts: its chakra and its will. He explained that whenever Naruto draws out the Nine-Tails' chakra, its will comes along with it. The Nine-Tails' will itself contains a mass amount of hatred, and it will relentlessly try to control Naruto whenever given the chance. To control its power, Naruto must have his own will and heart both very strong and free of hatred, in order to separate the Nine-Tails' will from its chakra completely.

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 covers the lock that keeps the Nine-Tails in.

The seal itself naturally weakens over time, something Minato created the key on the abdomen of the scroll toad Gerotora, to remedy. Gerotora explained that if the Nine-Tails' will is stronger than Naruto's while the seal is weak, the seal will break and the Nine-Tails will be released. Gerotora also explained that during Naruto's training, Jiraiya would normally use the key to suppress the Nine-Tails when necessary, but Jiraiya instead used it to weaken the seal for just a short time, to try teaching Naruto to filter out the Nine-Tails' will when he tapped into its chakra, but it instead unleashed the four-tailed form. 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 being released. 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. After Naruto gained control over the Nine-Tails' chakra, it changed to resemble the Sage of the Six Paths.

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
Naruto 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 pushed Naruto off of a cliff. In a desperate act to save himself, Naruto suddenly entered his subconscious and met with the Nine-Tails for the first time. Immediately, the Nine-Tails wanted to devour Naruto, but it couldn't because of the seal that kept it within its cage. Naruto, even though he realized what he was facing, demanded the Nine-Tails to give him some of its chakra as a form of "rent", in order to save them both. The Nine-Tails, realizing the situation, became intrigued with Naruto's bravery and complied, allowing him to summon Gamabunta.

The Nine-Tails then kept quiet till Naruto battled Neji Hyūga in the first match of the final round of the Chūnin Exams. After Neji used his Eight Trigrams Sixty-Four Palms to block Naruto's chakra points, Naruto called upon the Nine-Tails for chakra. Without hesitation, it gave him enough chakra to force his chakra points to open and gave Neji, with his Byakugan activated, a glimpse of itself. With the help of the Nine-Tails, Naruto was able to defeat Neji.

Invasion of Konoha arc
During Sunagakure and Otogakure's invasion of Konoha, while battling with Gaara in his full Shukaku form, Naruto and Gamabunta both did a Combination Transformation technique to take the form of a giant fox (the Nine-Tails itself in the anime). As the battle continued, Naruto called upon the Nine-Tails again for chakra in order to defeat Gaara, and once again, it did so without hesitation.

Search for Tsunade arc
During his search for Tsunade with Jiraiya, Naruto was confronted by Itachi Uchiha and Kisame Hoshigaki of Akatsuki. Sasuke soon arrived and attempted to attack Itachi with his Chidori, but the latter brushed it off and strangled Sasuke. When Naruto attempted to fight and called upon the Nine-Tails' chakra, Kisame used his Samehada to absorb all of its chakra. Before Kisame could cut off Naruto's arms, Jiraiya arrived and blocked Kisame's attack with his toad summon.

Later, before Naruto defeated Kabuto Yakushi with a complete Rasengan, Kabuto severed Naruto's heart muscles and the neighboring chakra system, which the Nine-Tails was unable to heal. While Tsunade desperately tried to save Naruto, the Nine-Tails noticed that its power was fading as its surroundings grew darker and darker. 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
While Naruto battled Sasuke at the Valley of the End, Sasuke, in his first stage of the Cursed Seal of Heaven, pierced a fatal wound in Naruto's right shoulder with his Chidori. Fortunately, the Nine-Tails quickly gave Naruto enough chakra to enter his initial jinchūriki form and healed his large wound almost instantly, and with that, Sasuke saw the image of the Nine-Tails for the first time. As the battle continued, the Nine-Tails gave Naruto more chakra to attain the one-tailed form, something the Nine-Tails indicated that he should be grateful for.

Rescue Gaara arc
During his battle against the Akatsuki member Deidara, which turned out to be a Clay Clone, Naruto slipped into his two-tailed form 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 his three-tailed form was proven insufficient in his fight against Orochimaru, Naruto gave into the Nine-Tails and entered his four-tailed form. The Nine-Tails gained control of his senses and began to attack everything that was 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 chakra, but he refused, saying that he doesn't need it or its power. The Nine-Tails just simply laughed and sarcasticlly mocked Naruto that he always comes to it anytime he is in danger, pointing out that he will be useless without it. The Nine-Tails tried to tempt Naruto to break the seal completely, so it can "grant" Naruto all of its power, but he still refused. Before the Nine-Tails could put up much of a fight, Sasuke, with his Sharingan activated, appeared within Naruto's subconscious. The surprised Nine-Tails was impressed with Sasuke's progress, but it commented that Sasuke's chakra is more ominous than its own, and that he is just like Madara Uchiha. After hearing this, 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.

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.

Hidan and Kakuzu arc
During Naruto's nature transformation training, the Nine-Tails' chakra was surfacing within some of his shadow clones that had grown very frustrated with combining the Rasengan with his wind element. Fortunately, Yamato was able to suppress the chakra before it can cause serious damage.

Invasion of Pain arc
During Naruto's training at Mount Myōboku, the Nine-Tails prevented Fukasaku from fusing with Naruto, wanting Naruto to use its chakra instead of senjutsu chakra. It was later 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 form. Despite the First Hokage's Necklace's attempts to suppress it, the Nine-Tails quickly seized the necklace and ultimately crushed it.

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 form was released. Before it could convince him to completely remove the seal, however, the spirit of Minato appeared in Naruto's subconscious and stopped him. Enraged, the Nine-Tails tried its best to kill Minato for revenge, but Minato just simply 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.

Fourth Shinobi World War arc
When Naruto was summoned back to Mount Myōboku, the Great Toad Sage gave Naruto Gerotora, along with the key to the Nine-Tails' seal. From this, Naruto was on a task to find a way to control the Nine-Tails, in order to fight a fated battle with Sasuke.

At a remote island in the Land of Lightning, after Naruto completed the first step at the Falls of Truth, Killer Bee then lead Naruto to a secret place behind the waterfall, saying it was there where Naruto would fight the Nine-Tails. When Naruto entered his subconscious, the surprised Nine-Tails questioned Naruto where is the "real him", due to not sensing any hatred within Naruto. After Naruto used the key to completely unlock the seal, the Nine-Tails forcibly pushed the cage's doors wide open and attempted to attack Naruto, but Killer Bee used the Eight-Tailed Giant Ox's tails to restrain it and help Naruto drain out its chakra. However, the Nine-Tails immediately sliced off the Eight-Tails' tails and drained part of its chakra. The Nine-Tails then bitterly condemned Naruto for joining forces with the Eight-Tails so he can try to control its power.

As the battle started, the Nine-Tails first fired a massive blast of chakra at Naruto, but Killer Bee used the last bit of the Eight-Tails' power to contain the attack. After Naruto was in Sage Mode, he used one of his three shadow clones to perform the Sage Art: Super Great Ball Rasengan on the Nine-Tails as a distraction for Naruto to come from behind and slam it to the ground. After that, two of his shadow clones attacked the Nine-Tails from above with the Wind Release: Rasenshuriken, causing great damage and giving Naruto the chance to drain out its chakra. However, the Nine-Tails planted its hatred within its drained chakra and it suddenly began to consume Naruto, making him lose the control and forcing him out of Sage Mode. The Nine-Tails roared at Naruto that he will never control it and to disappear forever, but Naruto's mother Kushina Uzumaki appeared, saying it is alright if he stayed. After Naruto realized who she is, his positive emotions and renewed resolve halted Naruto's transformation. Kushina then used her chakra to subdue the Nine-Tails in the form of chains, who quickly recognized her chakra and was greatly surprised by her appearance.

After being comforted by Kushina, Naruto's positive emotions overcame the Nine-Tails' hatred and he made multiple shadow clones and they charged at it, forming multiple Rasengan as they ran. After the Nine-Tails broke most of the chains that had been restraining it, Kushina immediately used the last chain on the Nine-Tails' leg to pull it backwards for Naruto and his shadow clones (all in Sage Mode) to attack it with multiple Super Great Ball Rasengan. As it was pushed back, many shadow clones pulled out the Nine-Tails' chakra as Naruto used the Wind Release: Rasenshuriken, weakening the Nine-Tails greatly to completely separate it from its chakra. When the chakra merged with Naruto, the enraged Nine-Tails attempted to fire one last attack, creating a sphere of chakra many times its own size, but with most of its chakra gone, the Nine-Tails became visibly emaciated. Its attack ultimately dissipated as Naruto used the key to bind the Nine-Tails with massive interlocking torii and resealed it. As it disappeared into the darkness, the Nine-Tails told Naruto that it will never forget this.

Influence
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 (possibly 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

 * 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.
 * The Nine-Tails seems to be fully aware of its origins, because when Naruto stole his chakra the Nine-Tails said it resembled the Sage of Six Paths, which also means that is very likely the other tailed beasts are aware of their origins as well.

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 Naruto) "Now then... Who do you want to kill...?"
 * (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 Sasuke) "... I never thought that you are able to suppress my power!... This may be our last meeting, but I warn you... Don't... kill Naruto... You will only... end up... regretting it..."
 * (To Naruto) "Destroy everything... Destroy everything that hurts you... Give your heart to me... I will save you from the pain..."
 * (To Naruto) "You even joined forces with the Eight-Tails... and call yourself a jinchūriki!? You should be ashamed!"
 * (To Naruto) "You don't have what it takes to control me! You're nothing more than a mere fragment of my hatred!"
 * (To Naruto after he took most of its chakra) "Naruto... don't... DON'T MAKE ME ANGRY, NARUTO!!!"
 * (To Naruto after its been resealed) "... I won't forget this... Naruto..."