I need a character to be invisible to the camera, but still reflect

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.