To reply more prosingly: I've been thinking about this a lot lately, because I'm working on a novel with a protagonist who isn't much like me, and perhaps even less like me at her age (16). She's physically brave, joyfully athletic, easy in herself -- all things I wasn't. I was strongly identified with my nerdiness: she's a pretty good student with a couple niche/bookish interests but also a varsity athlete and doesn't see any conflict there (I don't see the teens these days sorting themselves strongly in that way, when I substitute-teach.) I guess the bookish interests are ones I share, and some of her flaws -- she's awkward in some ways, and a little too snarky for her own good, along with other imperfections less true of me. Oh! And she has an unfortunate response to blowhards and jerks that is PRECISELY cribbed from me. But mostly...not much like me at all. Would she like me if we met? I hope so! In some ways I'm more like her mom, so yeah, she'd put up with me, with some eyerolling ;)
I'm not sure where she came from. This story needed a heroine, and she came out on the page. Sometimes I can just hear a voice and there's a person behind it, like her, but I usually have more idea where a protagonist came from, and they usually share a few more quirks with me than she does. I love her though, just as I do characters I make out of more of my own clay.
I think you have to have compassion or understanding for basically every important character in a book. I've actually managed difficult people in real life by imagining them as characters, even! Which means...I'm better at compassion for flawed human beings on the page than in the world. And that's important: when I do make a protagonist who shares my own flaws, I forgive and love her or him readily -- although in real life, there's no one I instantly take against more than someone who shows my worst traits in high relief. Writing's good practice for forgiving myself, I guess.
And my current heroine? That impulsive, athletic, brave girl? I know just who she is, and have no idea where she came from. I'm glad to know her though.