The aim is to have an AI you can only see using mirrors. Does anyone have any ideas?
This is just a stab in the dark, but you might want to look at the custom depth stencil buffer and render-to-texture. You can set up a post-process material that would render the character with a specific stencil ID as transparent for the main camera. Use a separate camera to render the character’s “reflection” to a texture, and then add that as a stencil to the “mirror” surface, or perhaps as an input to a material used on surfaces in which the AI can be seen. It might get a bit tricky calculating the proper camera placement to render the reflection properly for a specific surface, or if you have multiple surfaces, but it’s doable.