Kurama

Kurama (九喇嘛, Kurama), more commonly known as the Nine-Tails (九尾, Kyūbi), is a tailed beast currently sealed within Naruto Uzumaki of Konohagakure. The fox was first sealed into Mito Uzumaki after the battle at the Valley of the End, then into Kushina Uzumaki after Mito's death, before being sealed into Naruto when he was born.

Background
Kurama first came into being in the waning days of the Sage of the Six Paths, who used his Creation of All Things ability to separate the Ten-Tails' chakra from its body and divide it into nine separate constructs that would come to be known as tailed beasts in order to ensure that it would never resurface after his death. Some time after being created, the Sage sat down with all the young tailed beasts and told them that they would always be together, even when separated, and that one day they would become one entity again with different names and forms than they did. Then, when the time came, they would know what true power is. Over the centuries, Kurama has gained a reputation as an age-old natural disaster, appearing suddenly out of nowhere to attack areas that have been breeding grounds for the darkest aspects of human nature.

Kurama swallows Kinkaku and Ginkaku. At one point during the time of the formation of the shinobi villages, the Gold and Silver Brothers of Kumogakure were assigned to capture Kurama, but were both swallowed whole by the fox. However, the two brothers managed to survive and gain some of its power by eating the flesh of its stomach for two weeks, forcing Kurama to regurgitate them.

After that event, Madara Uchiha defected from Konoha and used his Sharingan to control and use Kurama to help him fight against Hashirama Senju, the First Hokage, in order to exact his revenge. During their battle, Madara combined his Susanoo with Kurama, increasing the latter's offensive and defensive capabilities. However, Hashirama was able to remove Kurama from Susanoo and released the tailed beast from Madara's control, subduing it in the process. Sometime after the battle, Hashirama deemed Kurama too dangerous to be allowed to walk free and Mito Uzumaki sealed the fox within herself, becoming its first jinchūriki.

Over the period of time which included the first two Shinobi World Wars, Mito kept Kurama at bay except for one occasion when the seal had weakened as she was going through childbirth. Toward the end of her life, Mito passed on this status and responsibility to another member of her clan, Kushina Uzumaki, who later became the wife of Minato Namikaze, the Fourth Hokage. Although Kushina was terrified with the responsibility of containing Kurama, Mito assured her successor that the fox's hatred was powerless against her love.

Twelve years before the start of the series, after the end of the Third Shinobi World War, the Third Hokage and his wife made preparations to ensure that Kurama would remain sealed within Kushina as she gave birth to Naruto. However, despite keeping her childbirth a secret, Obito Uchiha discovered Kushina's location, killing her escorts and managing to break the seal, seizing control of Kurama. Since Kushina survived the extraction, Obito tried to have Kurama kill her before Minato got her and their son to safety. Soon afterwards, Obito summoned Kurama into Konoha and ordered it to destroy the village.

While Kurama was rampaging in the village and decimating the shinobi forces that tried to repel it, Minato managed to place a Contract Seal on Obito during their fight to free the fox from the masked man's control. Despite no longer being in Obito's control, Kurama still harboured a deep hatred for Konoha for viewing it as nothing but a mindless beast and sealing it away for decades and resolved to destroy the village without orders from Obito. Before Kurama could annihilate Konoha, Minato summoned Gamabunta on top of the fox, and quickly teleporting 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 Kurama, planning to seal it back inside her before she dies. However, Minato, knowing that Kurama would revive without a host, and how great a threat the masked man is, came up with a plan to give Naruto the means to combat the masked man in the future when he decides to attack again.

Since Minato himself couldn't fully seal Kurama as it was, he used the Dead Demon Consuming Seal to seal its Yin chakra within the death god and then prepared the Eight Trigrams Seal to imprison the fox, along with its Yang chakra, within Naruto. Kurama realized Minato's intention and attempted to stop the process by killing Naruto while Kushina was weakened, but both of the parents sacrificed themselves to protect their newborn child and completed the sealing, with Kurama cursing the pair during its last moments of freedom.

In the anime, it was revealed that Kurama 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 Kurama is a kitsune with red-orange fur and red eyes, it possesses the upper-body structure of a human, complete with opposable thumbs on its clawed hands. During the last remaining days of the Sage of the Six Paths, Kurama was a young kitsune much smaller than its present-day self, but was still much larger than the Sage. As time passed, Kurama's size increased to be around the same height, if not taller, as the Hokage Monument. When captured by Madara Uchiha, Kurama had achieved a size larger than a massive plateau, and small mountain it was standing in front of. Later, despite shrinking a lot after having its Yin chakra sealed away, it still remained a massive entity and towered above most structures. When Naruto separated Kurama from its chakra, its entire physique became visibly emaciated, but it later regained its healthy look after consuming Naruto's chakra while he was using its own.

After having its cage opened by Naruto — who had assumed a new form — this is also reflected on Kurama, when Naruto transforms into it. In this form, the fox's whiskers become much thicker, and circular markings appear on its shoulders, stomach, as well as the front and back of its paws. Lines also run down its spine, arms, legs and tails. Its size was comparable to that of the other tailed beasts, standing a little taller than Gyūki when standing alongside it.