In my game the main character is almost always on the right (the few exceptions will start the player character on the right, but pan them to the center) while the left is reserved for NPCs.
To be honest, there wasn't a lot of thought around why this is. It mostly became the rule simply due to consistency since a lot of the early cut scenes I had implemented tended to have the edge of the map on the right side, I placed the bust there so that way it obscures less of the scene.
However, I've come to like it more. There are a limited number of drawn expressions and the player will see those same expressions many times from the main character throughout the game. The NPCs on the other hand are seen less often, so I guess in a "left to right" sense, I'm giving priority to NPCs.