The Star Spirits and the Pure Hearts are the typical "good balancing the evil" type of plot coupon. The Crystal Stars, on the other hand, were actually created by the Shadow Queen to amplify her powers during her conquest of the ancient civilization. They were taken from her by the original heroes and used against her. The Crystal Stars stand out from the Star Spirits and Pure Hearts by not being inherently good or evil; just as they were used by the Shadow Queen to aid in her conquest and eventual destruction of the ancient city, they were used by the original heroes to seal her away behind the Thousand Year Door.

I chose the Pure Hearts because Super Paper Mario is the only Paper Mario game I've ever played.