Simple and Clean (PLANITb Remix) sung by Utada Hikaru
The theme song of Kingdom Hearts Birth by Sleep is a remix of Simple and Clean by Utada Hikaru, called "Simple and Clean (PlanitB Remix)." The song used in the trailer of Kingdom Hearts Birth by Sleep is Fate of the Unknown by composer Yoko Shimomura.
The theme song is called dearly beloved No its called Simple And Clean
The song used is a remix of Simple and Clean. It is also used as the opening theme for Kingdom Hearts RE: Chain Of Memories.
The KH: BbS soundtrack was released in the Kingdom Hearts Birth by Sleep & 358/2 Days Original Soundtrackcollection.
I think the main theme is Sanctuary, like in KH2. Utada Hikari is artist.
Simple and Clean
The Kingdom Hearts 1 theme song is "Simple and Clean" and the Kingdom Hearts 2 theme song is "Sanctuary" (aka "Hikari" for Simple and Clean and "Passion" for Sanctuary). They keep those songs throughout the series.
That would be Utada Hikaru. She is the singer of the Kingdom Hearts theme songs. (Simple and Clean/Sanctuary)
The Kingdom Hearts II theme song is called 'Sanctuary', by Utada Hikaru.
Utada Hiraku
So far the theme song for all the kingdom hearts games is "Dearly Beloved" but they put a new twist on it every time.
Nobuo Uemastu.