I received a comment about this, so I'd like to clarify: this is a "reaction" novel (as the tags indicate).
In this format, characters in the universe react to a certain external element (in this case, the paintings), which in turn generates changes in the original story and thus continues the dynamic.
The protagonist is kept in the background by design, so don't expect him to become stronger, collect Infinity Stones, or anything like that.
I know the protagonist has more lines than usual in this type of novel, but that's an intentional decision on my part.
The protagonist can't be changed, but if you prefer, I can reduce his on-screen presence to focus more on the characters in the universe in question.
Let me know how you prefer.