I think my only answer to this would be: to have agency. That’s the only thing a protagonist really needs, and a goal that they would strive to succeed.
I really dislike books where the characters seem to have almost nothing going for them. And no reason why they should be the centre of it. If they have nothing they are going to die for, then it makes them less interesting.
So to make a compelling protangonist, it’s to give them a tough goal, sonwthing they want to do. Something they are willing to throw their lives away for. Even for a series, the way it can be achieved is to have small arcs which affects them, but still has relation to their goal. Or even talk about their own and reflect on what they have done. It makes them think and gain something from that experience. Which I think a series more or less needs.
As for a book, agency is what I think it’s more important rather than any typical traits. Because then they just become carbon copies of another character, not standing truly on their own. Even more than looks or personalities, as their eventual goal and what they go through to get it is what that’s going to change them. Because if I was dragged into something I didn’t want to do, I would be whining and moaning especially when there’s almost nothing why I should be involved with them apart from some ridiculous prophecy.
And they also have to be active, they would have to be on their toes all the time, and doing something which is the sole reason why I care. If they don’t do anything or go through some ordeal that changes them, then I would really have no connection to them. As I can’t see their character at all, or even understand them beneath. They are just one dimensional.
But anyway, let me know what you think of this post and leave them down. What makes a compelling protangonist and what makes me care for a character? This is just what I think it’s essential to make them more interesting than just flat. The chosen one trope gets a little tiring after a while, after all.