Those created without hearts exist as Dolls. The Dolls lack memories and personal thoughts and feelings, that is of course, until the missing hole in their hearts can be restored. They live day to day with restrictions at the hands of the Dollmaker. Some Dolls are free, others reside within the Dollhouse itself. 

Myungsoo is a Doll, one of the few male dolls, and he, like the other Dolls, has a fragmented memory. He writes in a diary to make sure he won't forget. He won't let himself forget the light, when there's already so much darkness clouding him and his memories.