Well I got one for each except for sidekick, but I might still get to that later.
I picked Spike for hero, because to me he's probably the most heroic character on the show, fighting not just forces on the outside, but the demon on the inside as well. No matter how much you try to put him down, he'll always rise up again.
No matter how much of a hero that Buffy can be, she's most definitely an anti-hero in s6, going dark, abusive. Yet despite that you see she's mostly still trying and clinging on to the hero she used to be.
Well Spike was newly souled, still innocent in the ways of a hero and in constant risk, first as a ghost and later on when he was corporeal, desperate to find a place to fit in.
I took the scoobies as a whole as the evil overlord, because there was nothing in the series, darker than the scoobies overcome by their own darkness.