in castlevania fighter appears carmilla, but I do not know who made the sprite, you should check with them and ask permission, but i dont know that concept carmilla want to use, as she wears a pink dress, but if you want to make a more gothic style, as the girl in the picture, is also very good idea would be a new concept, I could help with a sprite base, but only to give an idea, since I'm not long on pc
I think the Carmilla I'm requesting is not the same character from the actual Castlevania game but a different 'adaption' all together that still uses it's basis on Sheridan Le Fanu's novel well just like for another example would be a particular web series floating around with their own adaption of it...
And also remember she's also a playable character not a boss and I think you could use Stella as a sprite base or maybe creating a new one.
Of course her game play would play alot like or similar to Alucard from Symphony of the Night but except she's a actual vampire not a dhampyr therefore she's probably more powerful (rather her power increases overtime after going through the castle...) which I could imagine we get to access alot of the vampire powers that Dracula uses (well especially turning into a swarm of bats, shooting fireballs, having her own badass demon form which maybe could be more 'feminine' etc) making her 'his equal' sort to speak if not maybe even more powerful since she's a character who existed or written long before Dracula was in symbolic 'fringe brilliance' terms.
You know I was thinking that maybe Carmilla and Dracula are 'rivals' which they have opposing views which Dracula wants to destroy all humans over loosing his mortal wife, while Carmilla who has more common sense than Dracula and unlike him, she sees humans as very important source for feeding, but still sees them as equals especially since being potentials for future vampires if they really want it or worthy of it. '