Double D is in a rosary because his demon side had sealed itself from the world from the glitch powers it had to be cursed with to keep both of them alive. If you must know, It's the glitch from the Adult Swim short Learning with Pibby. Unfortunately he had to make a deal with the glitch or darkness to keep himself from going to Hell and to stay on Earth. The reason is that He had to do all of this is because of an Ouija Board scam Eddy had. Once it was tested out, Double D had was somehow afraid enough for the Ouija Board to strike him in the right eye out of anger. Instead of getting help by a priest, he rest it off. Once he got up and done his hygiene, he freaked out into the mirror because he was cursed as a demon. He's an incubus demon and he had no other option but to be dropped into Hell. But the unconscious deal with the glitch; I've mention before, somehow kept him on Earth. He had to stay jaded and emotionless because of the glitch, so it would make sense for him to seal his demon side into a rosary. Once he has the rosary on he will not remember the Ouija Board scam, the deal with the darkness/Glitch or the fact that he was ever a demon. Matter of fact, he doesn't even know why he has it on in his human form. That's why he's in a eye patch throughout the story. I'm going for the semi Hazbin Hotel/Helluva Boss reference when it came to Double D's design and backstory. I'm not good with making stories, but I still make them for fun.