Square Enix's an Disney's Kingdom Hearts series of video games takes place in an unnamed outer space-like fictional universe with numerous self-contained worlds to explore over the course of play. Many of these worlds are based on animated Disney movies, though Kingdom Hearts II introduced some worlds based on live action Disney films. In addition to the Disney worlds, Square included original worlds over the course of the series. The concept of the games and setting was developed by Tetsuya Nomura, the series' director. The series centers around the main character Sora's search for his friends and his encounters with Disney and Final Fantasy characters on their worlds. The first game, Kingdom Hearts, takes him through each world to lock their keyholes and prevent the Heartless from destroying them. In the Game Boy Advance sequel Kingdom Hearts: Chain of Memories, taking place in Castle Oblivion, he revisits memory based versions of many of these worlds. In Kingdom Hearts II, Sora helps the residents of these worlds again in search of his friend Riku. The Kingdom Hearts games have been both critically acclaimed and commercially successful. The worlds used in the games have received praise from reviewers.