Kyūbi Naruto

Kyūbi Naruto, also known as Kurama Naruto , was originally 's Final Smash in Super Smash Flash 2 prior to Beta 1.2, where it was replaced by Tailed Beast Rush.

Overview
When performed, Naruto gets on his hands and knees and slowly transforms into his Kyūbi form, releasing a powerful burst of fiery Chakra that deals 21% damage with decent knockback to opponents hit. Once it disappears, Naruto is temporarily transformed into Kyūbi Naruto. Kyūbi Naruto is one of the many "Final Forms" in the game, which means he is much different from his normal form and has greatly improved attributes, such as more powerful attacks and increased knockback resistance.

Origin
In the Naruto universe, the Jinchūriki  are ninja that at a very young age, are sealed with extremely powerful and monstruous entities know as Tailed Beasts or "Chakra Monsters". The immense strength that the tailed beasts possess is tempered by their bestial nature, which restricts them from being intelligent enough to use it effectively, thus when the tailed beasts are sealed within humans, they are able to benefit from the knowledge of their hosts, giving them the ability to better utilise their powers. Because of their power, they were sought out by countless humans throughout the ages, and were usually treated as mere tools to be used or taken, a sentiment they absolutely detest. In addition, they are well aware of their origin and the state of the ninja world.

The Jinchūriki are typically lonely people, viewed by those around them as the tailed beasts that had been sealed within them, leading to prejudice and being hated and feared. However, some Jinchūriki refuse to waver, even though they have been shunned, and they eventually earned respect. Such examples include Naruto Uzumaki, Killer B, Yugito Nii, and Gaara.

The Shinobi world has a total of nine Jinchūriki, all of them with a tailed beast sealed inside them, from the One-Tail, to the Nine-Tails. The Jinchūriki have shown to have access to the tailed beasts' massive chakra reserves and, by training, develop unique fighting styles and use the tailed beasts' considerable abilities: Gaara manipulates sand, Yugito breathes fire, Yagura materializes coral, Rōshi creates and manipulates lava, Utakata creates a corrosive alkali, Fū is capable of flight, Killer B produces ink, and Naruto has quick regeneration and can detect negative emotions. In addition, the Jinchūriki can use the chakra to transform themselves into the likeness of their tailed beasts, which further increases their abilities.

The Jinchūriki have the potential to control their tailed beasts' power, but require intense training and sheer willpower in order to properly wield it. Whenever the Jinchūriki draws out a certain amount of the tailed beasts' chakra, the beasts' very own psyche will fuse along with it, which usually involves an inner struggle between them as the Jinchūriki goes deeper into their tailed beasts' forms. If a Jinchūriki were to completely give into its influence, the seal which contains them will break and the tailed beast will be released. For the Jinchūriki to have complete control, they must find something to fill in the void of loneliness within their hearts to give them strength.

Trivia

 * Inside the data files of Super Smash Flash 2, there is an unused icon of Kyūbi Naruto for Classic mode. He shares this trait with Wario-Man, Super Saiyan Goku, Bankai Ichigo, and Crazy Hand.
 * Although Kyūbi Naruto uses a shadow clone to charge the Rasengan in-game, in the anime and manga, he did not need the help of a shadow clone to do this.
 * In v0.9a, there was a bug where Kyūbi Naruto could not jump, sometimes.