I understand what you’re trying to say here, but I am of the belief that not every single class has to have an outside impact on the game, or a new spin on everything. Some classes you just make because they’re fun, and they have a unique challenge of being thrust into a goal (in this case, of preserving someone whose identity you don’t initially know, nor their play style [both cases would indicate a need to adjust when you use your guarding ability, and what you do during the daytime.]
I also think that a Mercenary who can betray her target wouldn’t add anything new. We already have someone whom you are supposed to give your identity, but can already betray you. The Psychic.