tl:dr; Here are some tests of a character only visible in the mirror
So well before I commissioned that Jill Valentine mirror comic, I had been thinking about what happens to Riley after she eats Brad's soul. My current plan is that she wakes up herself again the next day, not remembering anything, but Eisheth is still inside her. Obviously the succubus is going to take over again, and it could be a typical TF sequence, but I wanted more than that.
I decided that there needed to be some sexy times involved (even if it's just with herself), but I also thought it would be fun if Eisheth could torment Riley in the process. I came up with the idea that Riley can see Eisheth in the mirror, but not in the real world. Eisheth would do some (naughty) things to her, culminating in Riley transforming back into succubus form and Eisheth taking over.
In order to accomplish that, and not drive myself insane with some kind of crazy post-work, I needed a way for my 3D tools to render a character but only in the mirror. Fortunately, there is just such an option available, and you can see the results in the render above. In the work-space, Eisheth is standing just behind the woman (like in the reflection), but once rendered, she only appears in the ray-trace.
You may recognize the woman as Hugo's character Britney. Figured I was due for some fan art! There is more to this (non-cannon) sequence, and there will be a Riley version too, both of which will be posted once finished.