When rendering most of the actors (characters and backgrounds, except generic ones) a special shader is used to handle semi-transparency overdraw and to support various effects.
You can override the default shader by assigning a custom one to
Custom Shader property available in the actor configuration menu.
Be aware, that the shader is expected to have several specific properties and render passes in order to work correctly. You can use the built-in default shader as a reference when writing your own. The built-in shader is located at
See Unity documentation and tutorials on shaders and graphics programming in general for more info on how to create and modify the shader and what you can achieve with it.