Turns out the Millenium Castle is a mansion that was renovated into becoming a school for the mentally challenged. Arc had a near death experience(which also gave her superpowers) and the TAs decided to send her to the Millenium Castle because they thought she had gone insane. Roa was just the chef of the school and asked Arc if she liked blood in her meat, Arc, in her insanity, misinterpreted that and forced Roa to drink her blood.