This isn't that simple. You need to decide at what point the character is deemed to be "behind" the wall, and so on.
One approach that springs to mind is -
1. Have a second ortho camera for all the 2d stuff.
2. Detach the player game objects from the sprites themselves.
3. Decide if you want the player to appear in front or behind whatever object is there right now, and move the sprite back/forward appropriately. Deciding this is probably not going to be that complicated if you think about it in 2D. Scale the 2D sprite as it would appear in the 3D camera, but scale it correctly so it appears as though its in the original z position not closer to the camera.
Whatever case you have in that example is a trivial case, think of something a bit more complicated - lets say a grid of walls. The player would have to be in front of some and artficialy behind others.