The Glory Funhouse Mirror

Kind of like an optical illusion, but it's not optical.

The one whose existence revolves around making another person they often spend time around look better. This second character might be similar to the Main, but is worse in comparison, as if to have the audience say "This guy's bad, but he could be a lot worse" and take more of a liking to the Main.

The Glory Funhouse Mirror can come in many different forms: an Idiot, Useless Secondary, Rat Bastard, and Loser just to name some.