An overweight friend exploited by an insecure female to make herself look better in comparison. Said female will often hypocritically gush about the fat foil's "voluptuous" body, intelligence and sense of humor and claim she doesn't get why men don't appreciate her friend's stellar qualities. Needless to say, she will unceremoniously dump the foil if the foil loses weight and becomes even remotely attractive herself.
Megan: Wow, Charlotte looks so curvy in that dress. I don't why guys aren't all over her. You know, she'd be considered beautiful if she lived in Victorian times.
Guy at bar: Yeah, I get it, she's your "fat foil." Can I have your phone number?