The thing is you could technically have multiple demi-human races to the souls RP and it wouldn't change anything. Just give them some backstory as to why they are relevant and how they came about.

Remember that in the Dark series of the Souls franchise, it has been stated that countless kingdoms have risen and fallen throughout time. We could easily make a case for Elves, Dwarves, Orcs, Goblins and the like being that they were descendant from a Lord or a Chosen Undead who's accumulation of souls warped his biology which was then passed onto his children. Or heck, you could have these races be a result of experimentation with souls and living beings. The Lion Warriors from Dark Souls 2 are a demi-human race (one of the first I might add), and it is heavily implied that they came about through human experimentation.