Vivi is a Final Fantasy character that makes his debut in Kingdom Hearts II. He originates from Final Fantasy IX and is the only character from said game to appear in the series, although he is more weak-willed here. He is one of Seifer's gang's members, and the only member not from Final Fantasy VIII.
Kingdom Hearts II
In the simulated Twilight Town, Vivi enters the Struggle competition. His data soon becomes possessed by a Nobody and he gains incredible powers. He easily defeats Seifer with his fast movement and speed and Roxas must battle him. After he is defeated, time will freeze and Vivi's data will reveal its true form to be a Dusk. After the Dusk is defeated, the real Vivi appears, wondering how he got here. Roxas later fights fourteen Vivis (created by Nobodies hacking the system) in a tunnel. The "real" Vivi appears, and after Roxas and company move away from the tunnel, it reveals its true form to be a Dusk. After that, Vivi will be seen with Seifer for the rest of the game.
Unlike the rest of Seifer's group, Vivi doesn't dislike Roxas and the others and is even quite friendly to them, always speaking in a polite tone and even offering Roxas his Struggle club when he was done training, although it is revealed that when Vivi does this he is actually a Nobody in disguise.
The oddity of his Struggle battle (such as his enlarging his Struggle Bat) may be a homage to the Black Magic that he uses in Final Fantasy IX. Furthermore, that Nobodies masquerade as him is a reference to his original Final Fantasy incarnation: in Final Fantasy IX, Black Mages were a race of magical beings created from Mist that did not possess souls. Thus most of them only lived a year before their bodies slowly shut down, but as the prototype Black Mage, Vivi had a much higher life expectancy. However, the truth of his origins still left Vivi questioning his existence and purpose in life. Various aspects of the Black Mage race are similar to the Nobodies of Kingdom Hearts.