Haven't watched the series for a while, but from what I can remember...
The series is set in a world where there a spirits that react to music and can get summoned by it, and the main character has made a contract with the red headed spirit when he was a kid.
Whenever they go into "battle" or any kind of situation where they need the power of the red headed spirit, he pulls out a mobile organ, plays a specific song and the spirit goes into her unsealed form, which is pretty much an AP version of herself.
Here are some before and after pics from Wiki:
http://en.wikipedia.org/wiki/Image:Corticarte_1.jpg
http://en.wikipedia.org/wiki/Image:C..._True_Form.jpg