More detailed information about this appeared in the Agranor library in the new beta version of the game (after 0.72.001).
But in short: you need to find a paper in the store that symbolizes a body, register it in Kimiko's secret place as the elf's body and apply to it the maximum amount of green things for the spirit of Celica. As soon as the indicator shows that the spirit of Celica is the main one, that very button will appear.