Maybe FANTASY MEDIATOR?

I think they could be interpreted a couple of different ways that are good, but for me I just have them all look like regular ole humans. They aren't a construct technically, they are humans with an ancestor who had no soul for lack of a better term. Now they don't have a soul, hence the can't cast thing. It could be really complicated. I like it that they aren't explained. Some DM might take it upon themselves to do some work and make a race like this fit. I like making opportunities for the FANTASY MEDIATOR, without designing the world for them.

It could get really long if we actually tried to come up with a coherant race origin and explaination.