Destiny Islands - Lite

Kingdom Hearts is a collaboration between Squaresoft and Disney, an action-RPG videogame.

From the Squaresoft Europe site:

KINGDOM HEARTS is a joint project between Squaresoft and Disney Interactive. The title not only features many classic Disney Characters and worlds, but also introduces new characters and environments created in the renowned Square Style. Featuring more than 100 Disney Characters, KINGDOM HEARTS also introduces new disney characters - Sora, Riku, Kairi and the heartless - designed by Square's Tetsuya Nomura, director and character deisgner of KINGDOM HEARTS. Nomura is best know for is creations in the breakthrough titles Final fantasy VII VIII and X

From the Manual:

The Destiny Islands are a beautiful paradise where children's laughter abounds. Day after day, Sora, Riku, and Kairi talk and dream of new places, new worlds beyond their islands. They ponder the usual questions that adolescents do- about the world outside, about their own existence. Little do they know a great destiny awaits them. One stormy night, a terrible darkness invades the Destiny Islands.

Thus does their journey begin...

Sora awakens in another world, in a place called Traverse Town. His hopes of seeing new worlds are realized, but his friends Riku and Kairi are nowhere in sight.

Meanwhile, Donald and Goofy have also arrived in Traverse Town. Tey have come from Disney Castle in search of a "key" and its owner. Their king, who has mysteriouly disappeared, has left them a cryptic message:

"Now, there's somebody with a 'key'- the key to our survival... We need that key." 

What is happening to this world?The "somebody" mentioned by the mising king turns out to be Sora. To find Kairi and Riku, Sora joins Donald and Goofy and embarks on an epic journey.

Riku is one of the main characters of KH.

From the official blurb:

He is Sora's best friend. He may seem cool and collected for his age, but he is far from the quiet type. Always curious about the unknown, he begins to question the small closed world in which he lives.

From the Journal (SPOILER ALLERT! ):

 After Hollow Bastion When Kairi lost her heart, Riku allied himself with Maleficent to save her. Riku was actually the rightful master of the Keyblade, but once he chose darkness over light, the weapon chose Sora instead. Ansem exploited Riku's weakness of heart and possessed him.