There's this thing called the "Hero's Journey" which is a really deep look into what creates a good protagonist. I can sum it up succinctly here and then if you want I suggest looking it up yourself. There's a lot of depth to it.
The structure of a protagonist's tale goes like this
Life > Something Happens that Changes their life >They receive mentoring (either by a mentor or of their own experience) > The hero makes a (usually) permanent change about themselves or something else > The hero lives their new life with their lesson learned and the day saved
In terms of Star Wars this is
Luke on Desert Planet > Aunt/Uncle killed by Empire/Leia's message incites him to join Rebel Alliance > Obi Wan trains him > Luke becomes a force user and uses it to destroy the Death Star > Luke now lives as a Jedi who will master the force
But wait, episode 5 is the same as well
Luke lives his (now normal) life as a Rebel > Attacked by the Empire > Luke trained by Yoda > Confronts Vader > Now Luke lives his new life without Han, without a hand, and knowledge of his father.
So as shown in Star Wars, in terms of plot, what makes a protagonist is that they are perpetually learning. Luke didn't stop learning after episode 4, he kept going. This is the most relatable thing for a protagonist because we as humans are always learning as well. I suggest you make sure your protagonist is always growing and adapting. Even in slice of life if you make their attitude towards something, even a minor thing, change over time, you will show something relatable.
The other thing that I think makes a protagonist is that there is something about them that is immediately appealing. Maybe just physical features, or a heart of gold, whatever. In like Cinderella for example, we realize she is gorgeous, good hearted (animals love her), and she has undue hate towards her so we feel sorry for her. All of those things make us immediately sympathize.