Kurama

The is the 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-Tails' resurgence, the Sage split up its chakra and used his ability to create nine separate, monstrous constructs of living chakra, which would become known as the tailed beasts. Over the centuries, the Nine-Tails had 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 one point during the time of the foundation of the shinobi villages, the Gold and Silver Brothers of Kumogakure were assigned to capture the Nine-Tails, but the beast swallowed them whole. However, the two brothers managed to survive and gain some of its chakra and power by eating the flesh of its stomach for two weeks, which caused the Nine-Tails to regurgitate them.

After that event, when Madara defected from Konoha, he used his Eternal Mangekyō Sharingan to control and use the Nine-Tails to help him fight against Hashirama Senju, the First Hokage, to exact his revenge. The battle was so great that it tore the earth's surface 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 one of her clansmen, 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.

While the Nine-Tails went on a rampage in the village, it found Minato's presence and quickly fired a Tailed Beast Ball at him, but Minato was able to teleport the attack to a far off location. The Nine-Tails was eventually released from Madara's control when Minato placed a Contract Seal on Madara, but not before it had decimated the shinobi forces that attempted to repel it. Before the Nine-Tails could annihilate Konoha, Minato summoned Gamabunta on top of the beast, and later teleported it to Kushina and Naruto's location, while letting its Tailed Beast Ball detonate away from the village. Soon afterwards, Kushina used her chakra chains to subdue the beast, planning to seal it back inside her before she dies. However, Minato, knowing that the Nine-Tails would revive 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 the death god before preparing the Eight Trigrams Seal to imprison the beast, along with its Yang chakra, within Naruto. The Nine-Tails realised Minato's intent and took the chance to attack Naruto when Kushina was weakened, but its attack was unsuccessful as both Minato and Kushina quickly used their own bodies to shield their child, piercing them instead of Naruto. 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 kitsune with red-orange fur and blood-red eyes, it 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'. During its attack on Konoha, the Nine-Tails was shown to be around the same size as the Hokage Monument, and then later, despite shrinking a lot after having its Yin chakra sealed, it still remained a massive entity as its claw dwarfed Minato and Kushina. When Naruto separated the Nine-Tails from its chakra, its entire physique became visibly emaciated, but it later regained its former look after it had been consuming Naruto's chakra while he was using its chakra.

Personality
The Nine-Tails is a malevolent entity with a natural cunning and intelligence, but it does have a distinct sense of honour 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. According to the Nine-Tails, it has watched over Naruto ever since it was sealed within him.

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-Tails indicates that they know each other on a personal level, but without any respect. Moreover, when Naruto resealed the Nine-Tails, it made an allusion to the Sage of the Six Paths (but it never finished its statement), which could also mean that it knew about the Ten-Tails.

Abilities
Being the strongest of the nine tailed beasts, the Nine-Tails is said that it can 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 roar into a powerful shock wave that can repel and destroy anything within its radius, increase its physical strength and speed, and fire Tailed Beast Balls. In the anime, it was shown to be able to create twisters and breathe fire.

During its battle with Naruto in his subconscious, it was shown that even though it had its Yin chakra sealed away, the Nine-Tails was more than powerful enough to overwhelm the Eight-Tails' attempts to restrain it (although Killer B stated that he didn't have full access to his own powers within Naruto's mind), and effortlessly block Naruto's Sage Art: Super Great Ball Rasengan with its tails. Naruto also noted that it was extremely fast despite its massive size. Later, even after most of its chakra was taken by Naruto, it still had enough to create a Tailed Beast Ball many times its own size. If its jinchūriki dies (causing the Nine-Tails to die with them), the Nine-Tails will later revive.

Even though it's extremely durable, the Nine-Tails, when caught off guard, can be weakened by attacks that deals with massive physical impacts, such as Naruto's senjutsu-enhanced Wind Release: Rasenshuriken and Sage Art: Super Great Ball Rasen Tarengan. It is also susceptible to control by the Mangekyō Sharingan, though only Madara Uchiha was known to have accomplish this great feat. Sasuke had also used his Sharingan on the Nine-Tails, but only to suppress its chakra.

After Naruto conquered his dark side, it was revealed that the Nine-Tails also has the ability to sense negative emotions, after it enquired about the "real Naruto". Naruto also gained access to this ability after gaining control of its chakra.

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.

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 realise that 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 realised 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. Realising 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 assistance. 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 neighbouring chakra system, to 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 then 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 B then lead Naruto to a special room within the secret temple 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 fired a Tailed Beast Ball at him, but Killer B used the remaining bit of the Eight-Tails' power to contain the attack. While in Sage Mode, Naruto had one of his three shadow clones perform the Sage Art: Super Great Ball Rasengan on the Nine-Tails as a distraction for him to come from behind, slam it to the ground, and had his two other shadow clones attack it 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 his 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 a new, stronger seal to imprison the Nine-Tails once again. As the Nine-Tails disappeared into the darkness, it told Naruto that it would never forget this defeat.

Shinobi World War Arc
While Naruto and Killer B continued their way to the battlefield, the Nine-Tails pulled Naruto into his subconscious, berating him for using its chakra so carelessly. Calling Naruto naive for thinking he could stop the war and bear the hatred by himself, the Nine-Tails tried once again to tempt him with power, but to no avail, earning Naruto some recognition. After the Nine-Tails reminded Naruto about the day he met Sasuke and how he was consumed by hate even then, it taunted Naruto about his inability to erase the hatred from others and the world. In retaliation, Naruto pinned the Nine-Tails down with a torii. While looking at the trapped beast straight in the eye, Naruto exclaimed to the Nine-Tails with great confidence that he planned to do something about both Sasuke and the war. As the Nine-Tails recalled Naruto's growth, it then condescendingly complimented him for finally learning how to stand up for himself. Naruto, unnerving the Nine-Tails with his trademark smile, said that maybe someday he would do something about its hatred as well. The Nine-Tails reproached him for believing that, but Naruto, determined as ever, vowed to the Nine-Tails that it would be worth a shot as he left, leaving the great beast speechless.

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 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 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…"
 * (To Naruto) "How stupid are you?! I'm the Nine-Tailed Demon Fox! I haven't fallen so far that a little baby can influence me! I'm a living mass of malevolence!"