"Guardians are suppose to protect the world and not fall in love with their guardian spirit, ya fool!"

"who the hell are you?" Amber asked.

Guadrians are chosen people with spirits that choose them, it is sometimes a hidden ability that can't be seen in the normal world. The portal between dream world and the real world are call doors. Red doors are red zones also known as danger zones meaning the entrance the dream world, while blue doors are the key to leaving the dream world. There are stages when a spirit first awakens trying to contact the guardian it's a confauing experience since the power of the guardian is pulled out to summon the spirit. 

The contract between a guardian and their spirit is permanent. Guardians chosen by spirits serve as people who are chosen to protect but also serve as what separates a spirit from a dream eater. 

Dream eaters often times are what become of spirits without a guardian, dream eaters are the result of the deceased that can't move on to the next world due to their own regret. In the case of Amber, Krystal is Amber's guardian spirit, thus Amber is the key to keeping Krystal's conscious intact. 

There seemed to be some confusion for the story, so hopes this helps clears things up a bit.