Mephistopheles from the Faust legend.
He's a despicable agent of Satan who exists solely for the corruption of souls, but he's completely blunt about it. He's no deceiver or trickster; he straight up tells Faust "Look, I can grant you whatever you desire on this earth, but at some point, you're going to burn agonizingly in Hell for all eternity with no hope of redemption. You are flat out selling me your soul. Are you alright with that? May I remind you that Hell really, really sucks?"
And then Faust agrees. Brilliant.
Not only is he straightforward, but he's also quite the gentleman. And I have to say...
Dat outfit...
Also, the characterization of Satan in Paradise Lost was phenomenal. He lures you in with this seemingly tragic backstory, this facade that he's been mistreated by a tyrannical God, and he gives these rallying, powerful speeches, but by the end, you'll realize that he's an irrational, self-righteous lunatic, who didn't even have a good reason before he tried to
kill the omnipotent creator of the universe using military force alone. It was very well executed, you just have to be careful how you talk about it. Going around saying "Yeah, Satan is really great." is a recipe for disaster. Especially if you're a Christian.