*jumps at the occasion*
That is if you don't mind this being (absolutely) non-romantic, but it can still be very sweet.
The idea I have when i read this is that he isn't her co-worker and that he doesn't even recognize her, allthough she may recognize him (eventually). He is a guest to the place she is working in, a wealthy guest, a warrior. And a long time has past.
His personality, while still a teenage boy was one of sweetness and caring, but now "sie" has become an utterly self-serving and domineering "bitch", who cares about little else but hirself and hir own well-being. Not recognizing her, or even remembering his life as a boy much, he falls into the genetic trap that is incest, and is attracted quite a lot to her, desiring her, and taking her.
Her personality should be very sweet and loving, and able to forgive nearly everything, and tolerate nearly everything. She will stay with him no matter what trying her best to make him as he she remembers him. Something very deep inside "hir" conciousness prevents "hir" from killing her, or even causing permanent harm.
Well this may be a little bit too much over the top for you, as it will likely have bondage, pain, noncon and everything else, but it fits almost nicely to an idea I was having before. (We can negotiate things that you will absolute refuse to do, but the rest is open for game.)