Radiant Garden also known as Hollow Bastion in this game, similar to Kingdom Hearts II this world plays as the home base for Sora, Riku and Kairi. Chronologically the world order is Riku, Kairi and Sora
When Riku and King Mickey first arrive in Hollow Bastion the town is under attack by the Heartless, after the two take care of some of the heartless a young women named Lightning fights some of the heartless but accuses Riku and Mickey for preventing her from fighting all of them. Riku has only been to the Castle of Radiant Garden and didn't know his way through so he splits up with King Mickey to try and find, as Riku continues walking he ends up in Merlin's house where he accidentally barges in and meets Leon, Yuffie and Cloud. After Riku introduces himself Leon realizes that this is the "Riku" Sora has been talking about on his journey, Riku explains to Leon that Master Xehanort is in the process of returning, before Leon has anything to say heartless begin to attack Merlin's house Cid and Aerith run away somewhere safe while Leon chases after some Heartless. Riku makes his way to the Castle and finds out that Cloud has been following him, he wants to do whatever he can to help Riku so Aerith doesn't get hurt. Riku reunites with King Mickey and the three come across, Maleficent and Pete who are trying to make a new χ-blade to concur Kingdom Hearts hoping Ansem the Wise has some kind of report.
When Sora, Donald and Goofy first arrive in Radiant Garden, Sora begins to turn into Ven with a faded Disney Town Membership in his hand, Sora, Donald and Goofy see Cid walking out of Merlin's house he tells Sora and his friends that Heartless attacked the house and he had to get out of there. Sora, Donald and Goofy fight off the heartless and Radiant Garden is safe for now, so Sora, Donald and Goofy will head back to the Gummi Ship for now, but Sora finds something that catches his attention a flyer Blits Ball Tourtament that can raise money for Cid to make a machine to destory all the heartless in the town. Sora decides to stay for now and reunite with some of his old friends like Leon, Yuffie and Aerith, when he walks inside Merlin's house he finds that nobody is home and Pooh's Story Book is left wide open, Sora figures that Riku might have visited Radiant Garden already so he goes inside the book to see him while Donald and Goofy wait in Merlin's house. After leaving Pooh's Story Book, Leon appears in Merlin's and tells Sora, Donald and Goofy the Hollow Bastion Restoration Comity is back in business for now and he needs Sora help wiping out all of the heartless.
Kairi arrives is the second person to arrive in her home world Hollow Bastion recognizing it as the castle Riku took her while being possessed by Ansem, she also begins to remember when Aqua saved her from the Unversed. Kairi Leon and Yuffie who were chasing after some of the heartless because they attacked Merlin's house.