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?
Now here’s a real trick which will separate out your protagonist from most and open the door to a truly successful story: don’t just have one circle in the middle of your page - have a series or at least three concentric circles. In the outer one, put a minor loss, an inconvenience, a frustration, an annoying absence. In the next circle, within that one, up the scale of the missing thing: make it a real so, a sadness, something that really hurts your hero. And in the very heart of the circles, write the thing that the character is absolutely dreading, the biggest possible hole or gap or emptiness that you think you can put in there.
Now you not only have the beginnings of a powerful character, you have the sparking point for a great story. Your tale will be about what happens with that central circle: will it eventually be filled? Or will it come true, in all its painful glory?
Suddenly, all that filing cabinet of details will spring into life: the details describe what your character has been, what he or she has done, what they look like, what they have. But the concentric circles give you an idea of WHY they are there, what it is that moves and motivates them.
Your story is then simply the HOW: how does the character move from where he or she begins to the end, which is when that circle is either filled victoriously or left gapingly empty.