She's the fat one that eats all of the food that the anorexites and the leader don't want to consume. She doesn't really help the situation; she's sort of a benign tumor.
Sometimes referred to as taf.
"Sure, Margaret! Gotta' keep your figure! *snort*"
Three girls are sitting in Starbucks.
"OHMIGOZ!! They put extra whipped cream on this frappacino. Hey, Taffies (The Tag Along Fatty's nickname), eat this shit up."
"Okay. *self conscience*"
"Yo wuttup Bill?"
"See those hot chicks?"
"Heh, you can go for the Tag Along Fatty. You get any weight on your shoulders with that relationship! Ha!"
"Bill that was an awful pun."