Object that is allways visible/Transparency sort of/x-ray

I want an object that is allways visible allthough it is behind objects. I have a person walking into a building. My camera is outside the building an I want it to see the person even as he is supposed to be behind walls etc.
I can’t set the materials of the bulding transparent because I don’t want anything else visible except the person.
I saw a tutorial a while back but did not need it then. I just can’t find the tutorial now.
I am rendering using OpenGL render (this is for an arch-viz).

In the object properties in the display panel you can enable x-ray. This is for the viewport so will also show when you use the OpenGL viewport render