Hi, I am having the same problem. I want the side picture to only display for the protagonist.
When I remove line 115 and line 116 in screens.rpy, it makes all the font in the right leaning instead of centred for all characters.
On top of this, how would I define in the beginning of the script which characters have side image or not?
That's intentional—I designed the starting point of text to never change during the game. If it changes, it causes significant eye-strain over long periods of time, since the user's eyes will be jumping unnecessarily.
That's why there's a purple circle when a character doesn't have a side image. If you still want to change the behavior, you will need to program it yourself. You can use the UseSideImage() function under the dialogue screen to conditionally change the xpos.
Defining side images for characters is done through RenPy, here is a link to the relevant documentation.