Sir Lancelot

The “Knight of the Lake,” famed as the strongest of the Knights of the Round Table.
His illicit love affair with Queen Guinevere drove Camelot down the road to ruin, tarnishing his legacy in the legends of King Arthur.
As he often fell into madness due to troubles involving Guinevere, he has a particular compatibility with the Berserker class. In a lot of ways, he's just a guy who pushed himself too hard.
He was born with numerous abilities and aptitudes that take him beyond the realm of “high-spec” right into “completely overpowered,” and none of them were any help at all in solving his problems. But while I could sort of sympathize with him, at the same time I couldn't help but feel a little bit of black joy that so many terrible things happened to such a handsome and talented person. But then the girls in class just looked at me in disdain, like, “Eww, so jealous, gross!” while screaming in shrill voices about how cool Lance-sama is, and… ahem. Sorry, I'm getting off track here.
Anyway, if he and King Arthur had just been able to have an all-out brawl to satisfaction on a sandy beach under the evening sun, so many tragedies might have been avoided. Well, there's no guarantee, of course, but the fact remains that whatever else happened that was the one thing that Lancelot's nature never let him do. He even tried going insane to change that part about himself, but just ended up causing the way-too-serious Artoria to misunderstand a lot of things instead. Really, it makes me wonder if his origin is “being a nuisance” or something.

Fate/Zero material: Fate/Zero Encylopedia