As you could see in the intro, Panzer, Rilix,
and Elise attacked the Holy Arc. A rescue plane took of just in time but seemed to be dammaged as well. So it crashed into the rooftop of Desire Mine, causing a cave-in. This accident nearly killed Arthur, Melody, Rodi, and Forte.

Forte then was posessed by an Evil Spirit and left the mines.

The three Spirits of the Arc inherited the
badly injured heroes to heal them.

Although this is risky for the sprits themselves it is the only way for them to stay alive. Arthur, Melody, and Rodi are bound together by the spirits and as former enemies they are now forced to work together.