This is amazingly well written. Kudos to you

So, first and foremost, I should have figured this was going to play out as it did, and considering rape is really... not my thing, I still have to admit that the writing itself is great, from a mostly objective standpoint. Were the characters ever to get more sexual in-game than the two kissing scenes, this is actually what I expect would have gone down whenever Golbez and Kain were away. It's incredibly descriptive without being overly eloquent and dragging things out, which is something I've noticed other writers who try to seem better than they are do, and for that I applaud you.

Something that comes to mind is the relation between Barbariccia's offer and the title. From the viewpoint of an honorable individual, what 'mercy' the fiend was presenting to Rosa was hardly mercy at all, since I've seen folks driven to suicide from situations like that, but then again, we also have to consider just what Barbariccia is. When one is the immortal, anthropomorphic personification of the very wind itself, it should be expected that one's mindset should be at least somewhat alien, and so by Barbariccia's standards, perhaps she was being as close to genuinely merciful as she could by offering Rosa a chance to live as her concubine.

Altogether, while I may not have 'liked' the scene itself, the story on the whole is great, even for a mere one-shot. You've got a talent for the earlier Final Fantasy titles, and I can't wait to see what else you can come up with.