That sounds really interesting, and like it's going to be a lot of work. I'm looking forward to seeing what you make of it. My Sephiroth muses have some widely divergant answers to those questions. For instance my Responsive Sephiroth has never had to say 'no', because Hojo made sure that the people most likely to approach him know that he's 'too dangerous' to have sex with. My Keeper Sephiroth on the other hand doesn't really believe he has the option to say no to any of his lover's desires once he's committed to a sexual relationship - although in the next chapter he has prioritised Cloud's wishes, even the ones he 'deduces' over Angeal's. My Chosen Sephiroth can't say no to Cloud, it's something genuinely impossible for him to do. In the Handler-verse 'no' isn't really an option for Sephiroth when he's dealing with his handler, but it isn't an option for his handler either, not when Sephiroth reaches the point of needing his handler's attention. And in my Pure Ds series I devoted a fic to discussing, among other things, what 'no' from Sephiroth would mean for them.