Life After Death
Everyone becomes a ghost when they die. That is the afterlife. After death the mind/soul detaches from the brain, travels into the next universe over, and forms a new body out of ectoplasm. Since it’s a natural process, it isn’t perfect, so some memories or even identity can be lost. The ghost could end up with no real consciousness at all, or with the personality and mind completely intact, or somewhere in between. The more humanoid the ghost, the more likely he had a quick death and a good recovery. Someone like Skulker likely lost a good deal of himself, possibly after a drawn out death. Johnny on the other hand was probably fully conscious when an accident cracked his head open and splattered his brains, causing an immediate and complete detachment.