IIRC - the system they used is not using 3d models for the environment in the end user seen end product. The environment is just mapped on flat planes, using color textures with normal maps and bunch of other stuff. And then viewed from suitable angle. It is bit like some “street art” where someone has painted a hole (or something) on the ground which looks like a hole only from certain viewing angle… you get the idea.
" I need to observe my character from the side, not from the top."
Your problem description seems bit mixed - you say you want isometric or orthographic something - then why don’t you use orthographic camera with proper angle?
If you want isometric or similar view angle, why not make it so? Surely, if you place your camera for top-down view, naturally your characters will end up seen from top-down view.
“How it is solved in pillars of eternity like game?”
I just wonder did you actually google for it? Actually, there was several videos… just google for “Pillars of eternity rendering system”
Edit: additional explanation in response to comment:
You’ll have to play around with this in 3d software to get the grasp of concept. I have no idea if this is exactly what they did, as I haven’t watched videos about making of Pillars of Eternity, I just once saw few seconds where they demoed the plane render tech, you’ll have to do the R&D work yourself.
I hope this image explains it.
The 3d rendering part (in my case) is done in 3d software. You’ll need to work that out yourself, it might be pretty complicated, if you need some buffers for shaders. Also, material definition in Unity might not be as easy as dropping materials, if you need occlusion culling or cast shadows on your “flat” plane, or some other depth based effects, as the depth is missing here. Having 3D meshes as occluders and shadow casters would negate the whole concept of efficient and low spec rendering. Texturing the plane is just the oldest trick in the 3d book - just take snapshot from camera angle and UV it back to plane.
Of course here I had to move navigation to side so that the effect is visible, navigation mesh would be where the textured plane is.