Basic Trope: Person A heaps abuse on Person B, but when Person C tries the same thing on Person B, Person A leaps to their defence.
Straight: Katie is constantly calling Bob lazy and fat, but when Gary calls Bob fat? Katie springs up, saying, "Hey! Only I can call Bob that!"
Katie never says a single good thing about Bob. Ever. But when someone remarks, fairly neutrally, "You're awfully tall..." Katie springs up, saying, "Nobody is allowed to talk about my Bobby-wobbykins that way but me!" And then gives said person a Megaton Punch.
All of Bob's so called friends do nothing but heap insults on him, but will ruthlessly attack anyone else says anything slightly rude.
Katie doesn't even let Bob says nasty things about himself.
Downplayed: Katie sometimes complains about Bob's weight, but gets angry when Gary insults him about it.
Katie knows Bob well, and knows he'd rather hear the tough truth than a white lie. But while Katie does it out of genuine concern, Gary is just trying to be insulting.
Katie knew Bob never took offense on her insults. After all, Bob had no qualms in dishing out barbs of his own.
...But then he realizes that no one else is ever honest with him about his flaws, leading to his interactions with other people becoming awkward. He and Katie reconcile, and she agrees to tone it down (while still being just blunt enough).
After the beatdown, Bob is taken to prison, and while Katie and Gary rejoice, only for Katie to beat up Gary because she should be the one to rejoice in Bob's sentence.
Bob comes to apologize to Katie, but Katie said that Bob was right and both are Not So Different after all. They became friends again and start to help each other with their flaws from that point on.