Basic Trope: Character is the main or sole reason that another character remains good.
- Straight: Alice keeps her older brother Bob's nastier side from surfacing.
- Exaggerated: The only thing keeping Bob from gleefully skipping across the Moral Event Horizon is Alice.
- Downplayed: Alice keeps Bob from being an aloof and condescending Jerkass.
- Double Subverted:
- Parodied: Bob is a psychotic, Spikes of Villainy-wearing Sociopathic Hero who turns completely harmless when Alice is around, and Alice refuses to believe he's anything else.
- Zig Zagged: Alice's presence prevents Bob from committing evil deeds half the time; it has no effect on him at other times.
- Averted: Alice's presence has no particular positive effect on Bob's behavior.
- Enforced: The director says he can't see why Bob is on the good guys' side when he's much meaner than any of them, so the writers add Alice to give him a reason to stay.
- Lampshaded:Bob: Now that the meddling fool is out of the way, I'm going to kill you!
Alice: Bob, what are you doing?
Alice: Good. If you hurt him, I'll be disappointed in you.
- Invoked: Alice was deliberately raised to be her brother's keeper and balance out his bad side.
- Exploited: The heroes set Bob up with Alice in order to keep him under control.
- Defied: "What, you thought I'd spare you because my little sister was around?"
- Discussed: "I always feel like Bob would kill us the moment our backs were turned if Alice wasn't around."
- Conversed: "He's one of those characters that only stays good because his sister is watching. I bet they'll kill her off or something eventually so we can see him really snap."
- Because she has to keep Bob in line all the time, Alice can't live out her own life and is just as chained to his side as he is bound by her existence. Mutual resentment gradually weakens the bonds between them until something finally gives way.
- Bob only allows Alice to moderate his behavior because his long-term goals hinge upon maintaining her trust and goodwill. Once his short-term goals are completed and You Have Outlived Your Usefulness, all bets are off.
- Eventually, Alice rubs off on Bob; he starts to become a genuinely good person even when she's not around, so eventually, he doesn't even need a morality chain to be a good person.
- After he thinks he no longer needs Alice, Bob starts misbehaving and surprises even himself when he still listens to her later. Whether by coming to sincerely value her goodwill and friendship, fraternal instincts overpower him or Alice stringing him along in some other way, she continues to moderate his behavior.
- Played For Laughs: Alice keeps Bob in line by terrifying him, despite being The Cutie in all other respects.
- Played For Drama: Alice is targeted by people aware of how important she is to Bob in order to get Revenge by Proxy.
The only reason I'm letting you live long enough to go back to Morality Chain is my sis, got it? Anything happens to her, I'll kill you, grind you into hamburger, and feed you to your mother, and if you're lucky, it will be in that order.