Holy buckets this is hard. There are so many choices.
Okay, here's a tie. These characters are hella different, so... that's fair, right?
Yossarian, from Joseph Heller's Catch-22
Yossarian is the shit. Catch-22 really should have been my "Favorite Classic Book," but I guess I wasn't sure about its "classic" status in comparison with Catcher. But whatever. I read Catch-22 when I was sixteen/seventeen, just because I FELT LIKE IT. Which was pretty empowering, for someone who didn't read a whole lot outside of class! But I loved it so so so much. I remember sitting around in school and reading it constantly. It was so great. The humor of the writing mixed with the tragedy of the story, so great. And above all, the characters all so colorful and weird and wonderful/horrible. I haven't read the book since then, but I've always meant to reread, and I always think back fondly on it. Yossarian succeeds in being so human and so ridiculous at the same time. Somehow he is the ultimate hero even though he defies everything that makes a hero. I don't know, it's been a long time and it's hard to explain. But I love him. He will forever be the imaginary absurdist rebel hottie of my heart.
Also! Not about Yossarian per se, but this book taught me many important things. Like the word "subversive"! How to make absurdist circles of dialogue! And how to do a fucking soul-shattering reveal with just the right kind of buildup that it destroys you and drastically alters your understanding of everything leading up to it.
That scene.... that scene. Guh.
Will, from Philip Pullman's His Dark Materials trilogy
These books, also, should have been on the list already. They could have been both the book(s) that makes me happy AND the book(s) that makes me sad. They could have been my favorite series. I don't know how I forgot them until now. Because I have a LOT OF FEELINGS ABOUT THEM.
I didn't expect to like Will. I first read The Golden Compass in 2002 or so, and loved every minute of it, but for some reason when I started The Subtle Knife I didn't get past the first couple chapters. I was just like, man, who is this Will guy and why should I care about him?? More Lyra please! Which is understandable I guess, but WILL IS AMAZING OKAY. I finally finally read these books properly last year, and I just about died, and I couldn't deal with how much I loved Will. I am finding it difficult to talk about characters without just talking about the books. Hmmm.
Welp, SPOILERS SPOILERS SPOILERS if you haven't read these books SKIP THIS PARAGRAPH:
I think what sealed the deal for me was Will losing his fingers. I am realizing that I have a HUGE literary kink for physical sacrifice/body transformation in the name of like... acquiring a new ability/maturing. That's such a weird and specific literary kink, and hard to explain, and hard to think of any other examples!! I guess a prime example would be mythology's Odin, trading an eye for knowledge. Fuck yes. And how Will dealt with the trauma of that experience, and his total badassery with the knife, and the evolution of his relationship with Lyra... oh god, those books were so perfect, and I can't go on or I'll start crying again.
Anyway, yes. If you haven't read either of these books, I highly highly recommend you seek them out.