They never really explain this but I think she transmigrated into Diana’s body as soon as Diana was born? Diana considers all of her memories to be her own when she describes anything. In chapter 22, Diana mentions that she had a really good memory of what happened when she was 10 (when her mother died) because she has memories of her previous life.
After reading those spoilers, am I the only one thinking that those guards are completely useless? Dave disappeared/got kidnapped, and was threatened twice, then Sylvia got kidnapped...what are the guards doing!? Playing pinochle in the other room? It's a good thing Richard removed those guys from Diana's guard, because if they're that incompetent, then she'd be dead by now.