Kurama

The is the most powerful tailed beast sealed within Naruto Uzumaki of Konohagakure.

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. After the battle, Hashirama's wife, Mito of the fūinjutsu-using Uzumaki clan, volunteered to have the Nine-Tails sealed within herself, becoming its first jinchūriki. Towards the end of her life, Mito passed on this status and responsibility to Kushina Uzumaki, who later became the wife of Minato Namikaze, the Fourth Hokage.

Twelve years before the start of the series, after the end of the Third Great Shinobi World War, the seal containing the Nine-Tails weakened as Kushina was about to give birth to her and Minato's son, Naruto. Despite keeping her childbirth a secret, Madara discovered their location, killed their escorts, broke the seal, and seized control of the Nine-Tails once again. After that, Madara summoned the Nine-Tails into the village, and ordered it to destroy Konoha.

During the attack, the Nine-Tails eventually became free of Madara's control when Minato placed a contract seal on him, however despite this, it had decimated many shinobi that tried to repel it. When the Nine-Tails tried to fire another chakra blast at the village, it was stopped when Minato summoned Gamabunta unto its back, and later teleported it away from the village to Kushina and Naruto's location, while letting the blast detonate away from anyone. Soon afterwards, Kushina used her chakra chains to subdue the beast, planning to seal it back inside her in hopes of it preventing from coming back for a while when she dies. Minato, knowing that the Nine-Tails would return without a host and a threat Madara still was to the village, came up with a different plan to give him and Kushina a chance to see Naruto again and give Konoha the means to combat Madara if he should ever attack the village again.

As it was held down by Kushina, Minato used the Dead Demon Consuming Seal to seal half of the Nine-Tails' chakra within himself, which caused it to dramatically decrease in size, but it still was large enough to tower over trees. The Nine-Tails soon realized what was happening as Minato prepared the Eight Trigrams Seal, and not wanting to be sealed again, took the chance to attack Naruto when Kushina was weakened. However, its attack was unsuccessful as both Minato and Kushina used their bodies to stop the Nine-Tails' claw from reaching Naruto, piercing them instead. The Nine-Tails cursed them for this before Minato sealed it into 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 fiercely intelligent, 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 shared the same fate as Naruto as long as it is sealed within him; so it had no choice but to cooperate with Naruto or it would die along with him. It would go to great lengths to preserve its own life, as shown when it forced its chakra into Naruto whenever his life was in danger. The Nine-Tails can also be very manipulative, as it was shown throughout Part II, it would relentlessly seize any kind of opportunity for it to usurp control over Naruto's mind and body, and tempt Naruto to completely break the seal.

Just like the Eight-Tailed Giant Ox commented that it did not like the Nine-Tails, the Nine-Tails itself had shown the same dislike towards the Eight-Tails, condemning Naruto for needing help from the Eight-Tails in order for him to control its chakra. 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.

Abilities
Being the strongest of the nine tailed beasts, the Nine-Tails' strength was said to create tsunamis and flatten mountains with just a single swipe from one of its nine tails. With its massive chakra supply, it can turn its simple roar into a powerful force that can repel and destroy anything within its radius, increase its physical strength to send destructive shockwaves, and can fire multiple spheres of concentrated chakra. During its battle with Naruto within his subconscious, it was shown that even though it had half of its chakra sealed away, the Nine-Tails was more than powerful enough to overwhelm the Eight-Tails' attempts to restrain it (though Bee stated that he didn't have full access to his own powers within Naruto's mind), and still had enough chakra to create a sphere of chakra many times its own size, despite having most of it taken by Naruto. The Nine-Tails' physique can even withstand the secondary effects of powerful attacks such as Naruto's Wind Release: Rasenshuriken and multiple Super Great Ball Rasengan, but still is weakened by the force of the impacts they caused. The Nine-Tails also has a unique sensory ability to detect negative emotions.

Activity inside Naruto
As the Nine-Tails' jinchūriki, Naruto is gifted with its enormous supply of chakra and stamina, immense strength, increased speed, accelerated healing (recovering from minor injuries within a few seconds to major injuries within a day), and, at times, a nearly impenetrable chakra shield. The Nine-Tails' chakra allows Naruto to perform the 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.

The Seal
The seal that imprisoned the Nine-Tails within Naruto is the Eight Trigrams Seal, which allowed a small amount of the Nine-Tails' chakra to always mix with Naruto's own. Whenever Naruto and the Nine-Tails conversed, it occured within Naruto's subconscious, which was 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. On the entrance gate of the Nine-Tails' cage, there was a parchment of paper that says, which covers the lock that kept the Nine-Tails in. Jiraiya later revealed that, when Minato sealed the Nine-Tails within Naruto, he had split its chakra in two, the Yin and Yang. Minato sealed the Yin half within himself, and sealed the Yang half within Naruto.

The seal itself naturally weakened 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 overpowered Naruto's while the seal was weak, the seal would break and the Nine-Tails would 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 weakened, Naruto's transformations occurred 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 could allow the Nine-Tails' chakra and influence to leak out of the seal, which would then rapidly progress to his four-tailed form, or onwards.

The seal also had two failsafes to prevent the Nine-Tails from being released: Minato would appear in Naruto's subconscious if Naruto transformed into his eight-tailed form, and would restore the seal to its original strength, but he could only do it once; and Kushina would assist Naruto if he attempted to control the Nine-Tails. After Naruto successfully separated the Nine-Tails' chakra from its will, the seal's appearance changed to resemble one of the Sage of the Six Paths' magatama, and Naruto was able to imprison Nine-Tails itself with a new seal that consists of massive interlocking torii.

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, Naruto and Gamabunta both did a Combination Transformation technique to take the form of a giant fox (the Nine-Tails itself in the anime), to battle against Gaara in his full Shukaku form. 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 legs, 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.

Land of the Sea Arc
In the anime, when Naruto fought Amachi in the Land of the Sea, after he realized Amachi has become a monster by utilizing the same research he used on Isaribi, he was overwhelmed by the power Amachi wielded at first. But as Naruto's desire to keep Isaribi from becoming a monster grew, the Nine-Tails immediately gave Naruto enough chakra to enter his initial jinchūriki form, and he quickly defeated Amachi.

Kazekage Rescue 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 offered Naruto some of its chakra, but he rejected it and told the Nine-Tails that he never wanted its assistance again. The Nine-Tails became contemptuous at Naruto, mocking him that if it wasn't for its power, he would be nothing. The Nine-Tails then tried to tempt Naruto to break the seal completely, so it can "grant" him all of its power, but Naruto 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 ability, but it commented that Sasuke's chakra was more ominous than its own, and that he was 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, for he would only end up regretting 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 absorb 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.

Confining the Jinchūriki 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 about where the "real him" was, due to not sensing any hatred within Naruto. After Naruto used the key to completely unlock the seal, the Nine-Tails pushed the cage's doors 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 quickly sliced off the Eight-Tails' tails and proceeded to battle Naruto.

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 was 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 chains to subdue the Nine-Tails, who quickly recognized her chakra and was greatly surprised by her appearance.

After being comforted by Kushina, Naruto's positive emotions depleted the Nine-Tails' hatred within its drained chakra, and he made a large number shadow clones, forming multiple Rasengan as they charged at it. After the Nine-Tails broke most of the chakra chains, Kushina immediately used the last chain that was attached on the Nine-Tails' leg to make it trip, giving Naruto's shadow clones enough time to enter Sage Mode and attack it with multiple Super Great Ball Rasengan. As the Nine-Tails was pushed back from the impact, Naruto and his shadow clones managed to grab and began to pull out its chakra as his one shadow clone used the 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 a sphere of chakra many times its own size, but with most of its chakra gone, the Nine-Tails became visibly emaciated and its attack ultimately dissipated as Naruto used the key to reseal the Nine-Tails. As it disappeared into the darkness, the Nine-Tails told Naruto that it would 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.
 * All the known jinchūriki of the Nine-Tails have been from the Uzumaki clan.

Quotes

 * (To Naruto) "I suppose I can't kill you, can I? After all, you die, I die. Your courage is 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! 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... Erase everything that hurts you... Give me your heart... I will save you from your suffering..."
 * (To Minato's imprint) "Fourth Hokage! Come here! I'll rip you to shreds!"
 * (To Naruto) "You even joined forces with the Eight-Tails... and you 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..."