To help things clear up, it's very clear that in RUIN, you are NOT supposed to know who the Mimic is yet. From Candy Cadet's story, you can understand the concept. It's very clear he's supposed to be a teaser, the first ever appearance of this new mysterious villain we'll get more info on later on! You do not need the books, so don't worry! :)
Ruin is probably my favourite dlc in Security Breach (Ik only one dlc but still better than the game itself) the story, the characters, the endings, I can’t wait what’s going to happen next, and your custom..are incredibly awesome! Especially with the broken Glamrock Freddy, can’t wait what you have for the next custom.
