Another way you could achieve this is by making a sprite object as a child centered to a parent cube, sphere, or w/e object with the render unchecked.
Then place a billboard script on the sprite object and then set up the cube, sphere, or w/e object as the 3D player.
This way the cube, sphere, or w/e object can rotate if needed (personally I would lock rotation) and the child sprite always faces the camera.