I may be interested in the idea, but I'm curious what you define as 'slave' for the sense of the RP? Like would he still be free to roam the castle and casually go about his day, but when she needs him for various reasons he is to come and assist.
OR like typical slave, she commands everything he does and says, he can't do much on his own without her command.
Just because I don't enjoy standard slave style RPs but could work with it if it is more like the first idea i mentioned.