Kyuubi

Also known as the Nine-Tailed Demon Fox, the Kyuubi is the strongest of the Tailed Beasts. Currently sealed within Naruto, the Kyuubi is a malevolent and cruel being with a habit of bearing powerful grudges upon those he believes have wronged him. He is an intelligent creature who uses his intellect to exploit any human weakness in order to seize control of his host’s body.