I came very late to the party with Buffy. I'd liked it, but my introduction to all things Whedon was Firefly, and I'm a Browncoat. So Xander, the one human being among the Scoobies, was the guy I rooted for. Sure, he had a thing for demons, and eventually ended up with Anya, but he was a construction-guy, a very normal sort, and he held his own, kinda, with the witches and werewolves, the slayers and potentials, hell, the vampires (alright, so he got his butt handed to him on a regular basis, and was kind of the cowardly lion of the Scoobies 8-).
What he had going for him was heart; he didn't let the fact that he was SOL 99% of the time stop him from saving the world on occasion. Yeah, exactly once, when Willow completely over-dosed on the magics and called up all of her power, tapped into it, and let it tap into her rage and grief. Xander talked her down, and Xander saved the world.
Kinda begs the question- what are you capable of, and what will you do, at the end of the world?