"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 Kingdom Hearts Re: Chain of Memories version contains the original melody played on the piano, along with a new 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.
The Kingdom Hearts 358/2 Daysversion has the secondary melody become the dominant melody, being played on flute woodwinds similarly to Roxas' theme. This is backed by harp and piano playing the original melody chord progressions.
The version of "Dearly Beloved" from Kingdom Hearts 3D: Dream Drop Distance has a relatively slower tempo throughout the majority of it, and has a solely orchestral arrangement. Unlike other versions, most of it is in 3/4 and it has a quicker section with a waltz-like feel in the middle.
The Kingdom Hearts χ arrangement has a varying tempo throughout. It starts off andante with a 4/4 rhythm, and later progresses to an allegro tempo. As with earlier versions, the piano dominates throughout, though a cello is heard at times during the piece.
In Kingdom Hearts 3D: Dream Drop Distance, during Sora and Riku's visit to Symphony of Sorcery, the duo work together to release the sound ideas that had imprisoned King Mickey. Together, the two create the melody of "Dearly Beloved". Mickey explains to Riku that the song was created by his and Sora's hearts, citing that the two are holding onto a little piece of each other.
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", "Birth by Sleep -A Link to the Future-", and "Dream Drop Distance -The Next Awakening-" 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". "Dearly Dreams", the field theme for the Mirage Arena in Kingdom Hearts Birth by Sleep, also contains the main melody from "Dearly Beloved", as does "The Key", the battle theme to Ventus-Vanitas. Most recently, "Link to All" from Kingdom Hearts 3D: Dream Drop Distance centers around the main melodic theme from "Dearly Beloved".