What Powers a Protagonist?
When working on a protagonist, you are building a construct, not a person.
By all means, assemble whole filing cabinets full of imagined personal data about the character. Many very successful writers operate that way. I once knew a wannabe writer who had such a filing cabinet, outlining even what socks his protagonist preferred to wear, all the data neatly tabulated in manila folders in drawers. And James Patterson, the world’s best-selling fiction author, has loads of biographical data openly available about his character Alex Cross. There’s obviously nothing wrong with that approach and it can be fun in its own right.
However, what motivates a character - and what attracts a reader - is not what is in that filing cabinet. It’s what is not.
In other words, right at the heart of a successful character is something or someone that they don’t have.
You could begin to construct a character by postulating first a hole, right in the middle of a page. A loss, an emptiness, a missing-ness, an unknown, a gap, a hole, a darkness. This could take the form of just about anything, from a loss of a parent (you will have noticed how many protagonists in fiction are orphans…) or a depressing school environment or a wound or scar or secret or mystery. Draw a circle on a piece of paper and leave it empty - what magnitude of thing is it? Has your hero got a secret? Or are they missing an eye or hand? Or are they pining after a friend or lover? Or are they trapped in some situation which is miserable for them?