What element makes a protagonist compelling?

As a character or as the story progresses? I'll answer both.

1, someone who is relatable, someone who starts off as the underdog, the weakest and slowly evolves into something stronger than their former selves.

2, this one is synonymous with the top answer, throughout the story, have the characters monologue out loud or have them interact in a way with other characters. Occasionally, just bring up moments that aren't necessary to the plot. Something that allows characters to bond with each other. A game maybe.
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Aaron Jones. Macabee Dean says of the difference between men and women: (regarding housework) “If it’s a man, he will not notice; if it’s a woman, she’ll say ‘This is the dirtiest _____ I’ve ever seen ‘!” Women are super critical of other women! Of course, we’ll criticize both sexes if it’s called for...😜
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People are complex. They are not only one thing, but shades of each aspect. You should shine a light on both the good and the flawed aspects. Protagonists can make mistakes. They can even do things that aren't especially nice or good or correct if they have a good enough reason or are conflicted or remorseful. You want people to connect with all your characters, so understanding their motivations, goals and thinking is important. Fundamentally, men and women are a good deal alike, Individual traits are distributed in bell curves, It's just the distribution of individual traits across the sexes that is different. The curves overlap, but are not identical. So, this means you can create whatever character you want. They just need to be consistent. But then all your characters need to be consistent. The mafia boss that thinks nothing of ordering a hit on some rival can still cry when his dog dies. And the girl who takes in stray cats off the street can summon great inner strength to endure some horrible tragedy. You get to decide who these people are - although some of your characters are likely to tell you who they are.
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My response might come across as a bit contradictory but here's my take...
I like protagonists who are imperfect but not useless. I like to see traits that I see in real people, depth and whatnot. E.g. everyone has that one crippling fear they simply can't cope with, everyone has that one defining trait that makes them unique.
Basically I like to see real people in a fictional world, if I read a book and the protagonist has traits that I can see in real life then no matter how flawed I can always connect with them.
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I want a character I can root for, who fights to overcome their obstacles. They can have flaws but I’d like them to recognize them and work toward rising above them. They can make one or two mistakes but I want them to learn from them and not keep making them.
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Characterization is everything. If the character isn't loved or relatable to the audience you lost the audience. TV shows that are spinoffs of movies are often because the characters are so well developed writers enjoy writing stories, challenges, and delving deep into the character's psyche. Hell, there have even been shows created for characters from movies, that were even just supporting characters because their personalities were so rich. Though the ones that come to mind are animated films like the Penguins of Madagascar, and recently Boss Baby, look at Shelden Cooper from Big Bang Theory. He was originally supposed to have been a supporting character and the focus was supposed to be on Leonard but who stole the show non other than Sheldon and now he has his own show. Personality and proper character development are key
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I'm always attracted to one of two types of protagonists: either the anti-hero type who has a dark past/much to atone for, or the hero who's almost annoyingly noble, and will make his/her decisions faithful to that nobility (watch the movie Rob Roy with Liam Neesan...great example).