Am expecting there to be many objections to this, but:
Cornelius Fudge.
When I first read the books I didn't see much in his character other than being a bit inept but with good intentions. But when I reread them (many, many times
,) I began to notice how power hungry the guy really is. We do know that he started out in a lowly Ministry position but aligned himself with powerful people and was soon elected Miniter of Magic. This indicates that everything he does is only to make himself look better. I mean, in GoF he went ahead and had Barty Crouch Jr. killed before asking any questions as a means of exploiting his power as Minister. I really see him as just a horribly blinded guy who will do anything and follow anyone to get power. Just think of how easy it would be for Voldemort to manipulate him without Fudge even realising it!
And finally, the fact that he refuses to act against Voldemort at the end of GoF shows that his loyalties don't lie completely with Dumbledore and the "good" side.