BEHOLD MY NECROMANTIC POWERS!
If we're talking about Jobbers... Hmmm....
Tatewaki Kuno is definitely one. The few times he gets a victory, he has plot armor, the element of surprise, his enemy is severely weakened, he's up against cannon-fodder goons, or he's teamed up with one of the other guys.
About the most impressive I've ever seen him was after the Watermelon Training, and in the anime, the few times he fights with Kodachi.
Not to mention, fannon has made him the ultimate jobber in that nearly every new character (at least those with any martial talent, which is just about all of them) winds up kicking his ass.
Essentially, he's like the "character test". If the character can't kick his ass, they're not worth following.
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I'd have to say Kazuharu Fukuyama from Girl's Bravo is definitely a Jobber. He's even been knocked out when the main character put his hand on Kazuharu's face (then again, considering Kazuharu has an allergy to guys, just about any guy could probably knock him out in such a fashion).
Plus, the guy's a creepy molester, FAR worse than even Ataru Moroboshi. He certainly deserves the ass-kickings he gets and more.
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The Efreet from Bastard! This guy was defeated by a FIRE SPELL. Efreets are MADE OF FIRE. Not only that, but the best he managed was to defeat the spirit beast in Arshes Nei's weapon in the anime.
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Adon, from Beserk. Seriously, I don't know HOW this fucker stayed a general when he lost... what, FIVE fucking fights, at least TWO forward positions (including a strategically important keep), and not only wasted a hundred of men in attempts to simply capture/kill Guts and Casca, but failed to ever do more than knock Casca off a cliff and shoot Casca with a crossbow.
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Fungus, from Final Fantasy Unlimited. Not ONCE does this guy win, even after surviving being blown up. Then again, I have to say, his final death... don't know why, but damn that was surprisingly chilling. I actually felt sorry for the poor guy.
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Naraku, Inu Yasha. I'm sorry, but this guy is a jobber. He has to flee constantly, his demonic BEES are more dangerous than him, and even when he DOES kill someone, it's indirect, thus not his victory.
Shit, I don't even count his supposed "victory" over Sango's clan to be one.
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Mizuki from Naruto. Never managed to kill anyone important. Ever. In the manga or the anime. He's injured Iruka, UNNECESSARILY killed an ally, and beat up a few NPCs. That about it. Shit, even when he was buff as hell and had Fujin and Raijin helping him, he never managed anything impressive.