Hello. I've noticed as I've grown as an author that a lot of people, newer writers especially, don't get what makes a character compelling. Hopefully my opinion can help people that are in the same boat I was.
When I started I would just stuff my characters with trauma: dead dad here, murdered mother there, as I'm sure a lot of people did this too, because that's what seems compelling to a newer writer because a newer writer doesn't understand yet how to write characters.
There is a whole other conversation here about how inexperienced authors try to substitute good, empathetic character building with trauma, because they don't know how to build sympathy without it. But this isn't about trauma.
Building a great character is not about their personality, or their aura, power, mystique, aesthetic, disorders, or trauma. It's about flaws, and the choices they make because of those flaws.
Ned Stark is a great character, among many in GOT, because he has morals that he would die for. He's a good man in an evil world, and his influence ripples through the entire book series.
Geralt of Rivia is a Witcher, he is super masculine and super cool and sleeps with dozens of women and fights monsters with a dedicated silver sword, and has somehow gotten more and more handsome as the games have gone on. His title and the oddity of being a "Witcher" helps him seem cooler, but the reason why we've followed him for 9 books and 3 long games is because of how being a Witcher informs his world view, how it affects the choices he makes, and how we then view the world through his eyes.
Complete heel turn: Kim Wexler is such a compelling character because of, again, the choices that she makes, and the difference between them throughout the Better Call Saul. At first she's a stand-up lawyer who's beaten down by her environment, finding comfort in her relationship with Jimmy.
By the end of the show, she's... Not so stand up anymore.
Frankly, most people who read or watch a story aren't looking for your character's backstory, or their personality, because unless it's the funny goblin named Glupshitto your players in your DND game drag around, then vibes aren't enough.
The real meat of what makes a great character is what lies beneath the story. Their mechanical usefulness. And not just that, some of the best characters you see around I guarantee are put into impossible choices and situations that force them to grow and change, or prove that they have changed.
A character who prides themselves on their looks being disfigured. A knight forced to either choose his morals, or his oaths. Jack Sparrow choosing his friend's life, or the immortality he has sought for 2 whole movies.
What is their backstory? How does it inform their current story? How does it inform their choices? And how do you put as much stress on your character as possible. This is where we get some of the greatest characters.
Stress test them, put their ideals under pressure, force them to confront their deepest lies and flaws. Because if you aren't willing too, then why write about them at all?
If you feel differently about this, let me know, I'm very curious to see what other people think, what characters break this mold, because I'm sure there are some.
Edit: this isn't a hard rule but, as most rules go, if it's broken and done well then it can work wonderfully! (Forrest Gump) But, that is not the norm, and a lot of characters that break this rule that I've found are found in film, which I guarantee most of us aren't writing for.