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Basic Trope: A character, usually but not always an older sibling, is so intensely overprotective of a sibling that they will not hesitate to severely punish or even kill a person who has hurt the sibling, often to the point of Disproportionate Retribution. If the sibling is dead, this can manifest as a Roaring Rampage of Revenge after the fact.
Bob explains that he takes his role as a protective older brother very seriously, and in his heart, Alice is the person he cares about most deeply.
Bob hates Alice's boyfriend for his own reason, and he uses his rudeness towards Alice as an excuse to beat him down.
Alice and Bob's dad is nowhere to be seen, so in his absence, it is up to Bob as to fill the role of the "man of the house" and watch out for his little sis.
Alice is protective of Bob.
Or: Bob is overzealous about protecting people from Alice.
Bob goes to give Alice's boyfriend a beatdown, but is made to listen to reason.
Or: Bob tells Alice's boyfriend that he's not happy, but that he'll let Alice deal with this her own way.
Double Subverted: Bob pretends to not care, but only to plot a more intricate vengeance in secret.
Parodied: A man accidentally spills a drink on Alice, Bob replies by loudly threatening him and his entire family in a comical manner.
Zig Zagged: Bob finds out about the bruises and makes a big show of declaring the boyfriend's doom...then he calmly calls the police. Before the police arrive at the boyfriend's house, the ambulance is carrying him out...but his injuries are because because he cheated some drug dealers...that Bob also knew well...
Averted: Bob's relationship with Alice is completely normal, and his protective feelings do not become particularly violent.
Enforced: "Sending Alice's violently overprotective brother up against her abusive boyfriend will make for a very brutal and dramatic scene."
Invoked: "If you so much as lay one finger on my little sister, I will make you wish you had never been born. Sure I might end up in jail, but sending you to the emergency room will be worth it."
Alice's boyfriend beats her, but Bob doesn't know and Alice refuses to tell him. Weak Carl, who cares for Alice but can't stop her boyfriend on his own, finds out and tells Bob. Bob effectively ends the relationship, and Carl can rest knowing Alice is safe.
Alice's boyfriend never laid a finger on her, but if Alice says otherwise, who's Bob going to believe? So Alice basically has a gun to her boyfriend's head at all times, and is not above occasionally reminding him of this when he's reluctant to do exactly what she wants.
"I hate his guts... but I must act within the law, rather than taking it into my own hands."
Alice tells her brother "I have grown up, I´m not your cute and defenseless little sister anymore, let me handle MY own life and stop following and protecting me like if you were my fucking babysitter, do you understand?"
Discussed: "Careful man...Alice's big brother is the crazy type who broke a guy's nose just because the guy called her a bitch. And only that because the mall cops pulled him away!"
Conversed: "Hmm, Alice's brother is the extremely overprotective type. If he finds out that Dave pushed her during that one argument, he'd better flee to another country while he still can."
Bob becomes so consumed with violently obsessive overprotection, that his sister disowns him.
He might end up seriously injuring or killing someone over a misunderstanding or something minor.
Reconstructed: When Bob delivers the No-Holds-Barred Beatdown, the boyfriend's Motive Rant reveals him for the monster that he is, showing that he is incapable of redemption and needs to be stopped.
Played For Drama:
Bob is obsessed over Alice so badly that he attempts Twincest with her.
Instead of acting like Alice's caring older brother, Bob acts like more like her Aloof Big Brother who is certain he knows better than she does how she should live her life.