"Dearly Beloved" is the opening theme to all Kingdom Hearts series games and is composed by Yoko Shimomura. "Dearly Beloved" is considered the main theme of the Kingdom Hearts series.
It begins playing as soon as the game loads and plays over the opening Menu Screen in a loop. However, if you continue to listen to "Dearly Beloved" without pressing any of the buttons on your controller for approximately 10-15 seconds, the screen will open up to Opening Menu Cinematic or Demo Loop, which will play the orchestrated version of the Kingdom Hearts theme song.
There is also a Reprise version that plays after the credits of the last screen in Kingdom Hearts and while viewing your Battle Report results in Kingdom Hearts II.
The original version of "Dearly Beloved" was composed solely on the piano.
The reprise of the original still contains the original melody, but this time combines piano and mallets.
The Kingdom Hearts: Chain of Memories version contains the original melody played on the piano, along with a secondary melody played in the background by a synthesized instrument.
The Kingdom Hearts II version transformed "Dearly Beloved" into a full orchestral piece (although the piano remains dominant), containing only the secondary melody.
The reprise of the Kingdom Hearts II version contains the secondary melody, but played only on the piano.
Kingdom Hearts 358/2 Days contains a nearly identical version of the Kingdom Hearts II "Dearly Beloved" reprise, only this time it has the flute used in Roxas's Theme.
The themes "Friends in My Heart", "Always on My Mind", "March Caprice for Piano and Orchestra", and "Fantasia alla Marcia for Piano, Chorus, and Orchestra" are variations of "Dearly Beloved" that can be heard in Kingdom Hearts. The "Concert Paraphrase on Dearly Beloved" in the Piano Collections Kingdom Hearts is also a different variation. "Musique pour la tristesse de Xion" from the Piano Collections Kingdom Hearts Field & Battle album, contains melodies from "Dearly Beloved". The field music for the Mirage Arena, "Dearly Dreams", is a faster, slightly more upbeat variant of "Dearly Beloved".