XIII機関
The Nintendo DS was chosen as the system before design work on the game had begun. Roxas was chosen as the game's protagonist as the staff thought that having a main character besides Sora would help to introduce gamers to the series' first Nintendo DS title. The staff thought that Roxas' role as a member of Organization XIII would be a suitable theme for the plot. This idea was furthered during the development of "Kingdom Hearts II", where the staff wanted to focus on what influenced Roxas to leave the Organization, which resulted in the creation of Xion's character who was connected with Roxas' origins. The development team wanted to have gameplay similar to previous "Kingdom Hearts" titles, but stated the number of buttons on the Nintendo DS was a problem. Some of the DS's functions, like the stylus, are not used in order to retain the similar gameplay from previous titles. Roxas seeing Sora's memories is incorporated throughout the game and has different types of gameplay for each world. The game's logo uses warm colors as a reference to the sunset in Twilight Town. The meaning behind the title, "358/2 Days" was left unexplained by Nomura during the promotion of the game. Upon completion, the player discerns that it references the story of Xion and Roxas, and their 358 days as members of Organization XIII.