Bowser from Super Mario Sunshine
Mario's nemesis, Bowser, has a new, creepy plan to seduce Princess Peach. He creepily convinces his son that Peach is his mother. When given this news, Peach is surprised but doesn't deny it, which is even creepier. Baby Bowser then kidnaps Peach, and the three of them head off for a creepy family reunion in a toxic hot tub in space. Mario has to stop them by cleaning up the brown waste Bowser and son have left in their wake.
What you'd expect ...
After the intense fire-based punishment of the preceding volcano level, you're more than ready for some one-on-one fisticuffs.
You expect, nay demand, a duel of epic proportions. You will settle for nothing less than Mario emerging from this final battle burned and bleeding, but triumphant, with Bowser's severed head in one hand and a swooning Princess Peach in the other, roaring his victory to the gods themselves.
Or, an updated version of those arenas where you swing Bowser around and throw him face first into a bomb would be good, too.
What actually happens...
Bowser's annoyed that Mario has ruined his vacation. He's not annoyed enough to actually get out of his damned hot tub, though. He just sits there, putting up a token resistance.
But he really doesn't give a shit and consequently is not only defeated, but humiliated in front of his son.
You can't really blame him. He's been at this for over 20 years. He must have known from the start that this would happen, and what good has fighting it ever done him?
So, the final battle of Super Mario Sunshine is an uncomfortable farce for all involved. After Mario has smashed Bowser's hot tub (yes, that's how you beat him) Bowser goes plummeting sadly to earth, where he and his son wind up stranded on a desert island. There, Bowser is forced to explain to his son that dinosaur monsters and human women cannot actually mate, Bowser enduring the kind of shame that would destroy any other man.
Bowser's son bravely endures his father's humiliation, and vows to get his own revenge on Mario when he gets older. King Bowser reassures him as the camera pulls back to reveal they are trapped on the island with no means of escape...
...and thus within mere days, Bowser will surely be forced to eat his son, ending his sad royal line forever.