Y'know, now that you mention Itsuki, I just thought of one character who might well be the ultimate jobber in anime.
Kyon, of Suzumiya Haruhi no Yuutsu.
Think about it: he wins and loses all according to the Will of Haruhi (in other words, the script), and neither happens due to his own apparent ability. He just kinda coasts along, doing what he's gotta do.
It could be argued that his very existence is a job.
If that's not the very extreme of jobbing, I dunno what is.