A long time ago, there were 12 guardians. Their job was to protect the worlds from the darkness, which sucks out the fun, cheer, and colorful atmosphere. One day, the darkness took over and the on,y way to bring the normal atmosphere back is to sacrifice themselves. 

The used all of their power to save the world, and disappeared. Years later, their spirit reminds unknown, but legend says that the power was givin to 12 girls, or the chosen ones.