@r-m-archer I’ve been studying flat arcs recently because I’m planning to write one myself. Basically a positive arc character learns a truth, but a flat arc character knows that truth already and has to stick to it throughout the story even when it’s difficult. The character doesn’t necessarily have to be noble, but they need a principle to stick to even in the face of opposition, so that they change the world around them in some way and the readers can root for them.
Flat arc protagonists are especially popular in more action-based stories, because there’s less time for in-depth character change and development in this kind of plot. Examples include Batman from the Dark Knight trilogy, Katniss from the Hunger Games, and Elanor from Sense and Sensibility.