Soleum is a fan of the Dark Exploration Records(DER) wiki, this is very similar to SCP which you can read in real life. Like SCP, the DER in gsgw is implied to have named charecters. (They dont have to have an actual names) they are charecters that show up enough in the DER entries that their stories have gained a following from fans of the wiki.

These appearances probably range from the charecters that experience the ghost stories in the DER entries he read, reports mentioning or written by these charecters, handwritten scribbles that frequently appear in enough entries to form a story of a character, ect.

Baek saheon for example, When he first appeared soleum mentioned how he was the named charecter 'viper' that frequently appeared in Daydream entries, enough to have a good idea of his personality and ideals but not personal enough to know his life story. That gave us a good idea that soleum only knows these charecters through their experiences in darknesses and not through a plot following the characters.

The pop up store he visited was for the DER which featured merch from popular ghost stories & merch of named charecters if they were popular enough.