The Kyuubi also known as the Nine-Tailed Demon Fox, is considered to be one of the most powerful tailed beasts. The Kyuubi is referred to as an age old disaster, a single swipe of its tail is able to produce tsunamis and level mountains. The Kyuubi attacked villages at random, apparently the Kyuubi had been attracted to places where human malice was high. The Kyuubi attacked the village of Konoha twelve years before the start of the series. None of the Konoha ninja were able to defeat the beast until Minato Namikaze, the Fourth Hokage, arrived. Minato sacrificed his life to seal the beast within his newborn son, Naruto, using the Dead Demon Consuming Seal. Minato did not completely seal off the fox's chakra, rather he sealed a portion off, allowing the remaining power to be accessed by Naruto.
Before sealing the Kyuubi away within Naruto's body, The Fourth gave a key to Jiraiya to either weaken or strengthen the seal on the Kyuubi if the need presented itself.
Jiraiya suspected that the Kyuubi's assault on the village was not coincidental and that it may have been summoned by Madara Uchiha, even though the thought of a single person summoning a beast of that caliber seems to be impossible,the fox seems to know Madara. In an encounter with Sasuke, the Kyuubi states that Sasuke's chakra is like Madara's, sinister. The Fox also seems to be familiar with the Uchiha clan, its relationship with the clan is as of yet, unclear.
The only present indications that Naruto is the nine-tails host is the whiskers which appear on his face and the seal which appears on his stomach when he is generating chakra. Due to the Kyuubi's chakra, Naruto is able to heal from wounds and fatigue in a very short amount of time, allowing him an incredible stamina. Naruto does not make use of the Kyuubi's chakra until he learns of its presence within him.
Naruto is able to call upon the Kyuubi's immense chakra to perform jutsus which are considered impossible for a ninja of his rank or stature. Naruto can access the Fox's chakra in two ways, one: he can submit to his rage allowing the Kyuubi's personality to take control making Naruto more blood thirsty and violent, or two:he can enter his subconscious mind and ask for the Fox for its strength. In the case of the latter, the Kyuubi is shown to have a loathing respect for Naruto and his father. While the Kyuubi usually grants Naruto his power out of amusement,he also does so to increase his influence upon Naruto. The Kyuubi also acts to preserve its own well-being,as in the case Naruto would die the Kyuubi shall along with him.
As Naruto draws upon the fox's chakra more and more, the Kyuubi begins to manifest itself in the form of chakra generated tails. Naruto can only maintain control over the first three tails, at the fourth tail he loses all rational thought and destroys or attacks anyone or anything nearby. In these cases, the fox's influence can only be suppressed by Yamato's Mokuton abilities or by a special seal created by Jiraiya to suppress the fox's chakra which is applied to Naruto's body.
In recent events, Itachi explained to Sasuke that anyone with the Mangekyou Sharingan can take control of the Kyuubi.
Different Charkra forms of the Kyuubi
0 Tail Naruto
There are two different forms of chakra for 0 tail Maruto. The first stage that has been shown is the more bestial stage then the other form. In this form Naruto's whiskers became longer, darker and more noticeable. His nails grow longer. His teeth start growing fangs and become longer. His jaw is widened. All of his previously injuries are healed at a remarkable rate. He also gets a power boost in this form. His speed, strength, agility, and charka, are all enhanced. Naruto's eyes also become red and the pupils become narrower like an animal. In this form Naruto is a full control but he is shown to be a lot more feral and animal like. He once actual went on all fours. It appears that this charka is triggered by rage.
The other version is when Naruto actual goes to the Kyuubi in his body. Naruto demands the Kyuubi to give him his charka which he calls "Rent" for the kyuubi staying in his body. The Kyuubi gives Naruto his charka out of respect for his courage to talk to him like that. Naruto body doesn't change at all but a red charka flows around his body. He gets a power boost but its hasn't been said which one of these forms are more powerful.
One Tail Form
This is the form after 0 tail form. This form is more powerful the previous form. Naruto has a lot of the qualities of the last form but this makes him looks more like the Kyuubi then the previous form. His body is engulfed with chakra that takes the form of a fox. The chakra that engulfs him appears to protect him from damage. That may be true but it also puts constant damage to Naruto body. So he fights while being constantly hurt and constantly healing at the same time. He is able to shoot a chakra hand that extends from the chakra that is surrounding Naruto. He is also able to perform Rasengan with the help of the chakra. Naruto is still in control in this form.
Two Tails Form
In this form Naruto has all the same abilities as the last. But of course he is more powerful. He looks more monstrous then before. His eyes are widened and the outside of eyes become black. His teeth become sharper and longer. His mouth becomes black too. He has two tails in this form. And the Kyuubi is more noticeable in this form. You see more similarities between the two in this form. Naruto still is in control in this form.
Three Tails Form
This form is more powerful then the last and Naruto has the hardest time keeping in control. He could lose control at any moment in this form. In this form the chakra bubbles a lot more then the last ones. He is able to counter a lot of his opponent’s attacks by roaring. In this form he has 3 tails.
Four Tails form
This is the latest form we have seen in the Naruto Universe. It is the most powerful out of all the other versions. In this form Naruto body is completely tare off of his body and the Kyuubi's chakra acts like a skin. In this form it has been stated that he is miniature Kyuubi. Naruto has absolute no control in this form. Naruto is able to all the same abilities as the last couple of forms but it appears to be harder for Naruto to move in this form. In this form just a swing of his arm can create destructive effects or a simply roar. He was able to create a ball of chakra and swallow it and fire it with explosive effects. Not only is this form Naruto most powerful it is also his most dangerous to himself form. Apparently this form can shorten Naruto Lifespan. While in this form every one of Naruto cells are being destroyed. But more cells are created at the same time. The human body has a limit to the amount of cells that they can produce in their lifespan. If Naruto continues to go into this form his lifespan will be shorten. According to Jiraiya as he reveals to Yamato, Naruto can still think in the Kyuubi form with 0 to 3 tails. However, if he reaches 4 tails, he is entirely consumed by the Kyuubi and cannot think for himself. He also do not consider anyone as his friend as it was shown hurting Sakura.
Powers and Abilities
Kurama is the strongest of the nine tailed beasts. Even with only half its power, Kurama remained strong enough to defeat five other tailed beasts at the same time.
Kurama has a great deal of brute force, reputed to raise tsunamis and flatten mountains with just a single swipe of a tail.
Even for a tailed beast, Kurama possessed massive reserves of chakra.