Kingdom Hearts is a series of action role-playing video games developed and published by Square Enix. It is the result of a collaboration between Square and Disney Interactive Studios. Kingdom Hearts is a crossover of various Disney settings based in a universe made specifically for the series. The series features a mixture of familiar Disney and Final Fantasy characters, as well as several new characters designed by Tetsuya Nomura.
We can find 19 Final Fantasy characters in the Kingdom Hearts saga.
In Kingdom Hearts II, Squall Leonhart, Aerith Gainsborough, Yuffie Kisaragi, and Cid Highwind reappear in Hollow Bastion/Radiant Garden as the "Hollow Bastion Restoration Committee" to rebuild the place after they came back from Traverse Town, with the addition of Tifa Lockhart, Sephiroth and Cloud Strife from Final Fantasy VII. Cloud is now looking for Sephiroth to defeat him. Sephiroth can be fought later in the game after clearing certain stages, while Yuna, Rikku and Paine from Final Fantasy X-2 are seen as fairies. Auron from Final Fantasy X makes an appearance in Olympus Coliseum hired by Hades to defeat Hercules, but he ends up rebelling against Hades and aiding Sora and his friends. In Twilight Town, Setzer Gabbiani from Final Fantasy VI is seen as the Struggle Champion, while Seifer Almasy, Raijin (Rai) and Fujin (Fuu) from Final Fantasy VIII appear as the "Twilight Town Disciplinary Committee" with Vivi Orunitia from Final Fantasy IX as their friend.
The characters that appear in Traverse Town also appear in Kingdom Hearts: Chain of Memories and its remake Kingdom Hearts Re: Chain of Memories for PlayStation 2.