Kurama

The is a 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 and used to create 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 had collected and festered.

At some point in the past, when Kumogakure tried to capture the Nine-Tails, it once swallowed the Gold and Silver Brothers whole, but they survived and managed to gain some of its chakra while they rampaged inside of its stomach for two weeks, which caused the Nine-Tails to regurgitate them.

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, Madara was defeated and was believed to have died. After the battle, Hashirama's wife, Mito of the Uzumaki clan, sealed the Nine-Tails within herself in order to keep the threat that it posed contained, 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 Shinobi World War, the seal containing the Nine-Tails within Kushina weakened as she became pregnant and gave birth to 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. Since Kushina survived the extraction, Madara tried to have the Nine-Tails kill her, but Minato arrived in time to save Kushina and brought her to safety with Naruto. After that, Madara summoned the Nine-Tails into Konoha, and ordered it to destroy the village.

During the attack, the Nine-Tails was eventually released from Madara's control when Minato placed a contract seal on him, but not before it had decimated the shinobi forces that attempted to repel it. Before the Nine-Tails could annihilate Konoha, Minato summoned Gamabunta onto its back, and later teleported it away from the village to Kushina and Naruto's location, while letting its Tailed Beast Ball detonate away from anyone. Soon afterwards, Kushina used her chakra chains to subdue the beast, planning to seal it back inside her before 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 Konoha the means to combat Madara if he should ever attack the village again.

Using the Dead Demon Consuming Seal, Minato sealed the Nine-Tails' Yin chakra within himself before preparing to seal the beast, along with its Yang chakra, within Naruto. The Nine-Tails realized Minato's intent and took the chance to attack Naruto when Kushina was weakened. However, its attack was unsuccessful as both Minato and Kushina used their bodies to shield their child, piercing them instead. Before the sealing was complete, the Nine-Tails used its last moment of freedom to curse the pair.

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 eyes, it also possesses the upper-body structure of a human, complete with opposable thumbs on its clawed hands. Out of all the tailed beasts, the Nine-Tails' form most closely relates to the Ten-Tails’. When Naruto separated the Nine-Tails from its chakra, its entire physique became visibly emaciated, and the flesh of its face began to deteriorate as it tried to charge the Tailed Beast Ball.

Personality
The Nine-Tails is malevolent, cunning, and fiercely intelligent, but it does have a distinct sense of honor and possesses a loathing respect for Naruto. However, the Nine-Tails is also treacherous, as it was shown throughout Part II, it would relentlessly seize any kind of opportunity for it to usurp control over Naruto and tempt him to completely break the seal.

The Nine-Tails has an intense hatred for Madara and the Uchiha clan’s Sharingan due to the fact that Madara had manipulated it to do his bidding in the past. In addition, the Nine-Tails possesses a deep grudge against both Minato and Kushina because they were responsible for its sealing and preventing its freedom.

Comments from both the Nine-Tails and the Eight-Tailed Giant Ox indicates that they know each other on a personal level, but without any respect. Moreover, the Nine-Tails' reaction to Naruto's new seal had shown that it knew about the Sage of the Six Paths, which could mean that it also had the knowledge about the Ten-Tails.

Abilities
Being the strongest of the nine tailed beasts, the Nine-Tails' strength is said to create tsunamis and flatten mountains with just a single swipe from one of its tails. With its massive supply of chakra, it can turn its simple roar into a powerful shockwave that can repel and destroy anything within its radius, increase its physical strength and speed, and fire Tailed Beast Balls. In the anime, it can also create twisters and breathe fire. 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 Killer 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 Tailed Beast Ball many times its own size, despite having most of it taken by Naruto. While the Nine-Tails is extremely resilient to most attacks, it is not completely invulnerable to the Sharingan's powerful genjutsu and techniques that can deliver massive physical impacts such as Naruto's Wind Release: Rasenshuriken and Sage Art: Super Great Ball Rasen Tarengan. The Nine-Tails also has the ability to sense negative emotions.

Activity inside Naruto
As the Nine-Tails' jinchūriki, Naruto is gifted with its enormous supply of chakra and stamina, accelerated healing (recovering from minor injuries within a few seconds to major injuries within a day), and depending on the amount of the Nine-Tails' chakra he's using, Naruto is also given an increase in strength, speed, along with an increase in his healing factor to such levels that he recovered from a Chidori strike in mere seconds, and at times, he is granted a nearly impenetrable chakra shroud.

The Seal
The seal that imprisoned the Nine-Tails within Naruto was 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 occurred 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 said, which covered the lock that kept the Nine-Tails in.

Because the seal itself would naturally weaken over time, Minato created the key, on the abdomen of the scroll toad Gerotora, to remedy. Gerotora explained that if Naruto was completely succumbed to the Nine-Tails' hatred, 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' hatred 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 or beyond his four-tailed form.

The seal also had two fail-safes 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.

When the Nine-Tails' Yang chakra merged with Naruto, the seal's appearance changed into a shape of one of the Sage of the Six Paths' magatama beads, along with a swirl design that resembled the symbol of Uzushiogakure. Naruto was able to imprison Nine-Tails itself with a new powerful 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 Uchiha, 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. Even though he realized what he was facing, Naruto demanded the Nine-Tails to give him some of its chakra as a form of "rent", in order to save them both. Realizing the situation, the Nine-Tails became intrigued with Naruto's bravery and complied, giving him enough chakra 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 Naruto enough chakra to force his chakra points to open and 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 was near to its conclusion, 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
While on his search for Tsunade with Jiraiya, Naruto was confronted by Itachi Uchiha and Kisame Hoshigaki of Akatsuki. Naruto tried to use the Nine-Tails' chakra to help him fight against them, but Kisame quickly used his Samehada sword to absorb it all and attempted to cut off Naruto's arms. Luckily, Jiraiya arrived in time to block Kisame's attack with his toad summon and was able to force both Akatsuki members to retreat.

Later, before Naruto defeated Kabuto Yakushi with a complete Rasengan, Kabuto severed Naruto's heart muscles, and his 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
During the first stage of Naruto's battle with Sasuke at the Valley of the End, Sasuke, in his first level 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 heal his large wound almost instantly and entered his initial jinchūriki form. However, as the fight continued, Sasuke was able to predict and overcome Naruto's attacks with his fully matured Sharingan. The Nine-Tails condemned the unconscious Naruto for being too weak, and gave him more of its usual 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, Naruto fought against Amachi in the Land of the Sea after he realized that the scientist became a monster by utilizing the same research he used on Isaribi. Naruto was overwhelmed by Amachi's power at first, but as his 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 the conclusion of 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 about Gaara's death. Immediatly, Kakashi Hatake placed Jiraiya's special seal tag on Naruto's forehead, restoring him back to normal.

Sasuke and Sai Arc
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 the nescient Naruto that he was the cause of Sakura's injury, which greatly shocked Naruto and resolved not to rely on the Nine-Tails' power again.

Soon afterwards, when Naruto encountered Sasuke again for the first time in almost three years, the Nine-Tails began to offer Naruto its chakra, but he firmly rejected its assistance. The Nine-Tails became contemptuous at Naruto, mocking him that if it wasn't for its power, he would be nothing. It then tried to tempt Naruto to break the seal so it can "grant" him all of its power, but he still refused. However, before their quarrel could escalate any further, Sasuke, with his Sharingan activated, suddenly appeared within Naruto's subconscious, which surprised both Naruto and the Nine-Tails. The Nine-Tails became impressed with Sasuke's growth and ability, but as it criticized that he was similar to Madara Uchiha, Sasuke coldly reacted by using his Sharingan to suppress the Nine-Tails back into its cage, to which it never thought that he was quite capable of that feat. The Nine-Tails concluded that this was probably its last meeting with Sasuke, so it warned him 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. When 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 it from taking control of his body as he threatened the beast to back off. The Nine-Tails begrudgingly pulled back, but it soon managed to absorb 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 could 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 necklace's attempts to suppress it, the Nine-Tails quickly seized the necklace and ultimately crushed it.

When Naruto was contained by Pain's Chibaku Tensei technique, the Nine-Tails tempted Naruto to entrust 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, the spirit of Minato appeared in Naruto's subconscious and stopped him. Enraged, the Nine-Tails tried to dare Minato to come closer so it could rip him to shreds, but Minato simply ignored it and restored the seal to its original strength.

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 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 chamber behind the waterfall, where Naruto would fight the Nine-Tails. When Naruto entered his subconscious, the Nine-Tails expressed surprise as it could no longer sense any hatred within him, and questioned Naruto about where the "real him" was, to which Naruto replied that the real him was right in front of it. 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 then proceeded to battle Naruto.

As the battle started, the Nine-Tails first fired a Tailed Beast Ball at Naruto, but Killer Bee used the last bit of the Eight-Tails' power to contain the attack. Naruto, in Sage Mode, 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. Just before Naruto was completely succumbed, his mother Kushina Uzumaki appeared, halted Naruto's transformation, and used her chakra chains to subdue the Nine-Tails, to which the Nine-Tails reacted in anger and shock when it recognized her chakra.

After being comforted by Kushina, Naruto's positive emotions depleted the Nine-Tails' hatred within its drained chakra, and he made a large number of shadow clones, forming multiple Rasengan as they charged at the Nine-Tails. 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 bombard it with the Sage Art: Super Great Ball Rasen Tarengan. As the Nine-Tails was pushed back from the impact, one of Naruto's shadow clones immediately used the Rasenshuriken to weaken the Nine-Tails further and completely separate it from its chakra. When the chakra merged with Naruto, the enraged Nine-Tails attempted to fire a Tailed Beast Ball, but its attack was ultimately dissipated as Naruto used the key to reseal the Nine-Tails. As the Nine-Tails disappeared into the darkness, it told Naruto that it would never forget this defeat.

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... Entrust your heart to me... I will save you from your suffering..."
 * (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 it’s been resealed) "I won't forget this... Naruto..."