The series centers on the character Sora, a boy who searches for his lost friends and encounters Disney and Square Enix characters as he travels between worlds. In the first game, "Kingdom Hearts", Sora fights against the villains of the series known as the Heartless and seals each world he visits to prevent their return. 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 and the design of the worlds has been praised for its faithfulness to the source material.