Kingdom Hearts is a series of action role-playing games developed and published by Square Enix (formerly Square). It is the result of a collaboration between Square Enix and Disney Interactive Studios, and is a crossover of various Disney settings based in a universe made specifically for the series. The series features Disney, Final Fantasy, The World Ends with You, and Pixar characters, as well as several original characters designed by Tetsuya Nomura. In addition, it has an all-star voice cast which includes many Disney characters' official voice actors.

The series centers on Sora, his friends, and their encounters with various Disney and Final Fantasy characters along the way. Players primarily control Sora, though there are numerous characters that join his party as computer controlled members. Most characters were introduced in the original game Kingdom Hearts, with subsequent installments featuring new original, Disney, and Final Fantasy characters. Dream Drop Distance introduces characters from Square Enix's The World Ends with You, while Kingdom Hearts III introduces characters from Pixar franchises.

Various types of merchandise modeled after the characters' likeness have been produced, including figurines and jewelry. The characters have garnered praise from several video game websites and magazines for the quality of their voice acting and visual style. Comments have focused on the accurate presentation of Disney characters, the unique visual style of Square Enix characters, how well all the characters blend together, and the consistent quality performances from voice actors.

The Kingdom Hearts series is directed by Tetsuya Nomura, who is also the games' character designer. Nomura has stated that unlike working with Final Fantasy characters, keeping the main character Sora alive and interesting over multiple games is a challenge. He also stated that though many of the Disney characters are not normally dark and serious, there were not many challenges making them so for the story, and despite this, their personalities shine because they maintain their own characteristics. Though Disney gave Nomura freedom in the characters and worlds used for the games, he and his staff tried to stay within the established roles of characters and boundaries of the worlds. When deciding which worlds to include in the game, the development staff tried to take into account worlds with Disney characters that would be interesting.

The inclusion of specific Final Fantasy characters was based on the opinions of fans and the development staff. Another criterion for inclusion was whether the staff felt the characters would fit into the storyline and in the Kingdom Hearts universe. Nomura was hesitant to use characters he did not design because he was unfamiliar with the background of such characters. For Kingdom Hearts II, he changed his mind after receiving pressure from his staff. Throughout the development of the games, Nomura has often left certain events and connections between characters unexplained until the release of future games. Nomura did this because he feels that games should have room for fans to speculate and use their imagination. He stated that with speculation, even though a game may get old, people can still be happy with it.

Sora (ソラ) is the main protagonist of the Kingdom Hearts series, and the sole playable character in the original game. He is portrayed as a cheerful 14-year-old boy who lives on the Destiny Islands with his childhood friends Riku and Kairi. When the Islands are attacked by creatures of darkness known as the Heartless, Sora comes into possession of a weapon of light called the Keyblade, and he embarks on a journey alongside Donald Duck and Goofy to protect other worlds from the Heartless, Nobodies, and other forces of evil. Nomura has stated that Sora's name can be interpreted as "sky", as the Japanese word for sky is sora (空).

Riku (リク) is a childhood friend and rival of Sora who resides with him and Kairi on the Destiny Islands. He originally appeared as a non-playable character in the original game, where he is introduced as a 15-year-old boy, and has a playable role in the "Reverse/Rebirth" story mode of Chain of Memories, the multiplayer mode of 358/2 Days, and the core story of Dream Drop Distance. He is also a computer-controlled party member in the final level of Kingdom Hearts II, and is temporarily playable during a sequence where Sora is incapacitated in the game's final boss battle. Riku is also briefly playable at several points in the "Dark World" in Kingdom Hearts III and in its Re Mind expansion. His signature weapon is the "Soul Eater" sword, a manifestation of the darkness in his heart. During the first game, Riku is revealed to be the rightful owner of Sora's Keyblade, which Terra bequeathed to him during the events of Birth by Sleep; however, his reliance on the power of darkness caused the Keyblade to reject Riku in favor of Sora. In Kingdom Hearts II, where Riku is now 16 years old, he acquires another Keyblade of his own, the "Way to the Dawn"; this Keyblade is later broken in Kingdom Hearts III, leading him to obtain a new Keyblade, "Braveheart".

Riku also appears in Super Smash Bros. Ultimate as a spirit.