Pierre Anthon is basically everything Agnes is not: he's unafraid to be himself, he doesn't care what anyone else thinks, and he has no problem living up to his role and antagonizing the holy heck out of everyone who walks by. It doesn't get much more antagonistic than spitting plum pits at folks and telling them they might as well be chimps.
What he has in common with Agnes (and everyone else in the book) is his general obnoxiousness: these aren't characters you cozy up to. Pierre Anthon is there to serve as a mouthpiece for existentialism, and he does a fine job to be sure. You won't leave this book not knowing that everything, especially meaning, is relative.