"When people say they want a relatable protagonist, they really mean they want a resonating protagonist." It's close enough to a quote I heard in an old review.
I'd say the simplest step to seeing how to make a more likeable protagonist is to just go back and watch shows or read books with protagonists you like. Then watch them while taking notes about what it is that makes you actually like them. Are they the Man with a Plan that makes a difference no one else will? Is the reason other characters like him just because "he's nice", or is it because he goes out of his way to help and empathize with people without expecting anything in return? Is he really just a cool guy that wins every fight, or is he someone constantly struggling against his own limits to just barely scrape by each encounter, learning and growing from each beating along the way?
The thing is that you could take any story with a critically acclaimed protagonist, rewrite the same story in your own voice, and find that you have zero attachment to your version of the character you already love. It is way too much nuance that goes into something as broad as "interesting", so much that it's hard to fully explain without an entire thesis paper. There is the overall plot and main progression of the story that drives them, but then there's all the little touches along the way that humanizes them. It's learning when and where to insert those touches (not like that) that spice up a character and makes you care about how things branch out for them.
---nuance.
broad strokes are one thing. what about the tiny things.
---dichotomy.
a tough grizzled veteran of violence is one thing. are they something different when its not game time.
---little motif's or themes you pick up on.
I have a recurring character that has a habit, an uncontrollable nervous tic. Actually several.
the reader can sort of "read" him, through the tics. his tics are a sort of "barometer" of how uncontrollable/important things are at the moment
---something original, no matter what that is.
one good start, IMHO, is the abandonment of "alpha" hero OP MC. Surrounded by "beta" beggars.
there's other personality types out there, and some of them are downright scary.
I see a lot of one sentence, surface level take on things.
show the reader what's under the surface take. the layers that produced the final product you see.
I like juxtaposition. I'm not saying I pull all these things off like a pro. But I like to imagine I'm getting better at it, each attempt.
