OK here's one way to do this:
For a start, you need a way to recognize when your character moves behind an obstacle, or a way to figure out what lies between the camera and the character. Then, it is a matter of changing the material of the obstacle into a semi-transparent material.
One way you can acquire the obstacle is by casting a ray. I suggest you look it up by yourself, because as far as I know, there are multiple ways to work with rays in Godot. Choose the one that best fits your needs.
Take a look at the following sources:
3) and an older one: https://www.youtube.com/watch?v=y6yyDJy0uVE
Having said that, I have tested the following minimal example and it works: I have attached a script to the Camera node. This script casts a ray from the position of the Camera to 1000 units in the z direction of the Camera (again, there are other ways of casting rays that are more efficient, check the docs). If said ray detects a collision object (such as an Area or RigidBody), then it acquires the collider's MeshInstance using the
collider method. Afterwards, The script sets the collider's MeshInstance's material into another semi-transparent material using the
set_surface_material(0, mat) method.
onready var wall = get_node("/root/Spatial/Invisible_wall/Invisible_wall_mesh") # our obstacle
onready var mat = preload("res://Materials/orange.tres")# our second, transparent material
var space_state = get_world().direct_space_state
var result = space_state.intersect_ray(translation, Vector3(translation.x, translation.y, translation.z-1000)) # Cast a ray from camera
if result: # if collision is found
print(mat) #get the collider's MeshInstance and change its material
A few notes:
1) Raycasting works with Area and Rigidbody nodes
2) I have hardcoded everything in the example, but you can avoid that easily
3) A more efficient way to do this might be to attach the script on the colliders instead of the Camera node. That way you can set the logic in such a way that once the ray no longer collides with your obstacles, your obstacles can have their previous material.
I hope that helps!