Kurama

九喇嘛 / 九尾の妖狐

The Nine-Tailed Demon Fox is the most powerful of the tailed beasts and is also regarded as the "King of the Tailed Beasts." Its strength is so immense that it is viewed as an ancient natural disaster; a single sweep of one of its nine tails can generate tsunamis and level mountains. One of its unique traits is access to nearly limitless sources of chakra.
The demon fox's attacks appeared random, with the only discernible motive for its appearances being an attraction to places marked by human malice. It attacked Konohagakure twelve years prior to the start of the series. No ninja in the village could defeat it until the Fourth Hokage, Minato Namikaze, arrived to handle the threat. Minato sacrificed his life to seal the fox into the body of his newborn son, Naruto Uzumaki, using the Dead Demon Consuming Seal. However, rather than completely sealing away the fox's immense power, Minato only sealed a portion of it, leaving Naruto with access to the remainder for unknown reasons. Before performing the seal, Minato gave a key to Jiraiya to either strengthen or weaken the seal if necessary. The seal naturally weakens over the years, and if it is broken, the demon fox will be released; however, the bond between Naruto and Kyuubi, as well as the general bond between a Jinchuuriki (host) and a tailed beast, is such that if the seal is broken, Naruto will die immediately afterward. Furthermore, if Naruto dies while the seal remains intact, Kyuubi will die with him. Consequently, Kyuubi uses some of its chakra to boost Naruto's healing abilities and will occasionally "take over" (lending Naruto its chakra without his request) to keep him alive.