Spirit Hearts
Spirit Hearts are special hearts that the player can give to the 
Island Spirit once they've achieved 
Best Friend status with an Island Resident.
The player is tasked with healing the Island Spirit by giving her 
Spirit Hearts. With each Spirit Heart, the dark matter that surrounds the Island Spirit is slowly disappears.
After the completion of the "Absence Makes the Heart" quest, each following day, the player can bring one of their Best Friend Island Residents to the Island Spirit. In order to do this, the player must have reached maximum friendship with the Island Resident, indicated by a golden heart, and the Island Resident must be able to be the player's companion.
The following is a list of Island Residents the player can obtain Spirit Heart status with:
Badtz-maru, 
Keroppi, 
Retsuko, 
Chococat, 
My Sweet Piano, 
Hangyodon, 
My Melody, 
Hello Kitty, 
Cinnamoroll, 
Pompompurin, 
Pochacco, 
Pekkle, 
Wish me mell, 
Kuromi, 
Tuxedosam
The following is a list of Island Residents the player cannot obtain Spirit Heart status with:
TOPHAT, 
Big Challenges, 
Kiki, and 
Lala