There's an attractive girl who is generally nice, helpful and pleasant to be around. Her brother, however, is a whole other story: he's got the menacing aura around him. He may be a Perpetual Frowner, or easily irritable, or even have the Face of a Thug (or be actually ugly, up to Gonk levels). People would likely hesitate to talk to him, or to his sister when he's around.
The brother here is usually the older one, but rarely you may find a menacing younger brother. In many cases, he's fiercely protective towards his beautiful sister; occasionally, however, he may be a Big Brother Bully instead.
Sister Trope of Ugly Guy, Hot Wife, which is the couple version of this (and probably the reason why they got children as this). Related to Beast and Beauty and both Men Are Tough and Women Are Delicate. See also both Big Brother Instinct and Little Sister Heroine, and The Ugly Guy's Hot Daughter which applies to father and daughter instead of brother and sister. See also Sibling Yin-Yang.
Despite the name, this shouldn't be confused with Bizarre Sexual Dimorphism (but may be related in some way.)
- Slam Dunk: Hanamichi Sakuragi is in love with Haruko, a lovely schoolgirl who likes basketball, which makes him enter to the basketball club of his school to get a chance on her. Sadly, the leader of the basketball team is her big brother: Takenori Akagi, who's feared even between his partners because of his big size and Patrick Ewing-like features (Sakuragi calls him "Gori" for a reason).
- Hajime no Ippo shows Ippo's afections for the cute Kumi, a nurse who also likes boxing and supports him in his fights. But one of his most difficult fights is against Ryou Mashiba, a dreadful and ugly boxer known for hurting severely his opponents. Things Gone Horribly Wrong for Ippo when he discovers Kumi is Ryou's little sister. After the fight, both boxers become friendly rivals, but Ryou likes to stalk and scare Ippo for dating his sister.
- In Ryoichi Ikegami's Spider-Man, Yu Komori meets Rumiko, a cute schoolgirl. Sadly, at the end of the chapter Yu discovers Rumiko is the little sister of Electro, the first supervillain he fights as Spider-Man and even more sadly, the discovery occurs few moments after Yu killed Electro in the process.
- Inverted in The Elder Sister-like One, where Yu's older sister (while appearing to be a beautiful young woman) is actually Shub-Niggurath.
- Demon Slayer: Kimetsu no Yaiba: Gyutaro was hated by everyone, even his own mother, because of his countless birthmarks. His sister, Ume, however, was said to be beautiful from an early age.
- That Time I Got Reincarnated as a Slime has Souka and Gabiru. While they were initially introduced as lizard monsters, Souka transforms into a cute girl with wings after being named by Rimuru, while her brother remains lizard-like. Maybe justified in that Gabiru was already named by a much weaker being before encountering Rimuru, so he can't be re-named to receive a more humanoid appearance.
- The Excel Saga manga plays with the trope for all it's worth with Sumiyoshi and his sister Canal, who "talks" like her brother despite the lack of resemblance. Sumiyoshi's friends spend the entire chapter speculating about the genetic implications: their father's a Hot Dad with the "family voice", while their ''mother' is the Sumiyoshi lookalike, with normal speech balloons.
- Downplayed twice in My Youth Romantic Comedy Is Wrong, as I Expected:
- First with Hikigaya siblings, Hachiman and Komachi. The former isn't exactly ugly or feared by everyone else, but Komachi certainly is a cutie that has nothing to do with his brother, but Hachiman has a Big Brother Instinct over her, also inherited to Rumi Tsurumi and Iroha Isshiki.
- Second with Saki Kawasaki, who isn't ugly either but she's certainly feared because of her loneliness and looking at first sight as a Japanese Delinquent. But she's as protective of her little siblings as Hachiman is with Komachi.
- Black Moon Chronicles: Minor characters Goum and Nasha were separated during childhood, a mage experimenting on Goum to turn him into a gigantic gladiator (but blocking his intellect at childlike levels), while Nasha became a Little Miss Badass who once held a knife to a man's throat and was perfectly prepared to use it. Wis eventually manages to reunite the two, fixing Goum's intelligence in the process.
- One Hellboy story begins when a woman asks a witch for help in conceiving a child. The witch gives her a healthy flower and a withered one, telling her to eat only the pretty one. The woman does so and gives birth to a beautiful girl, and of course eats the ugly one and gives birth to a troll-like girl. However, the two sisters love each other dearly, and when the human one gets her head replaced with a cow's, her sister goes out and beats the everloving crap out of the trolls with a wooden spoon.
- Chumley and Tananda from Myth Adventures form a Red Oni, Blue Oni pair. Chumley is an urbane, thoughtful sasquatch-like monster. His sister is a short-tempered, impulsive drop-dead gorgeous woman who appears human except for her green hair and a slight greenish tinge to her skin. It's eventually revealed that they are Trolls - male Trolls are "Trolls" and female Trolls are "Trollops".
- Inverted in The Never Mythos short story My Old Friend Gideon the title Gideon is an Affably Evil Humanoid Abomination and his sister "Lili" is a hideous Eldritch Abomination described as a "drooling mass" and devours humans.
- In Mistborn, Quellion - also known as "the Citizen" - is a dictator in charge of Urteau who executes people for their lineage. His little sister Beldre is an innocent civilian and Spook's Love Interest. This is ultimately subverted when it is revealed Beldre is an allomancer, instead of her brother as Spook assumed, and her hostage situation was a ploy to sabotage the rebels' plans. Subverted again when Quellion is revealed to have been subjected to Mind Control by Ruin.
- A Song of Ice and Fire:
- Viserys Targaryen and Daenerys Targaryen. He's a cruel, arrogant bully who looks down his nose at everyone and has delusions of grandeur, with his little sister being his main punching bag. Although not ugly, Viserys has a gaunt, severe and occasionally unhinged look to him. Dany starts out as meek yet kindhearted; she later starts standing up for herself and doesn't shy from more ruthless decision-making, yet retains her compassion. She's also considered very beautiful and dainty-looking. After Viserys' bullying goes too far and he threatens to kill a pregnant Dany, her husband kills him as she looks on.
- Joffrey Baratheon and Myrcella Baratheon. He doesn't look monstrous, though his looks are about his only positive trait and fools several people, especially his betrothed Sansa. He starts out as a Royal Brat who thinks his position means he's free from any consequences, and is later revealed to be a probable sociopath who swiftly becomes The Caligula upon ascending the throne, enjoying physically and psychologically torturing people. Myrcella, on the other hand, is lovely inside and out (although she later gets half her face disfigured, it doesn't seem to negatively alter her personality that much), and is sweet and gracious to everyone. Many people cant believe how different the siblings are; it's telling that their mother blatantly favors Joffrey while side-lining her other children.
- In Warrior Cats, Mothwing is a pretty and kind she-cat who becomes a medicine cat, while her brother Hawkfrost is noted as looking dangerous as soon as he first appears in the books. It's later revealed that he wishes to follow in their evil father's pawsteps, and to do this he manipulates her to use her position to his advantage and plans a coup.
- Played with in Hemlock Grove: Roman is "normal looking", while his younger sister Shelley is gigantic, one-eyed, and "monstrous"-looking. However, Shelley is a very nice and gentle person, while Roman is a violent inner monster.
- Billy Kane is a fearsome character from Fatal Fury and The King of Fighters series, a sadistic punk with a large cane who's The Dragon of Geese Howard and Arch-Enemy of the "Hungry Wolves" team (the Bogard brothers Terry and Andy, and Joe Higashi.) His little sister Lilly Kane, however, is a motherly-like younger sister who awaits for his brother when he's in fights ignorant (or not) of what's her brother is doing and always worried for him. She's also hinted in different appearances to be Joe's Love Interest, which irritates Billy to no end.
- Ann's brother Gary from Harvest Moon 64 qualifies. He's not monstrous by any stretch of the word, but he's a "Social Disaster" as Ann puts it. Cold, silent, prone to bluntness when he does speak, and clearly doesn't like it when Pete takes an interest in Ann. That said, if you take the time to get to know him (which takes some work, believe us) it turns out that Gary is simply Quiet, not really mean.
- In Yooka-Laylee and the Impossible Lair, it's revealed that the respective Big Bad and Big Good of the game, Capital B. and Queen Phoebee, are brother and sister. Capital B. is hulking and obese, resembling a giant bee that gained human appendages, while Queen Phoebee is slim and much more conventionally cute.
- In Fire Emblem: Three Houses, there is Mercedes, a friendly devout healer and an all-around Nice Girl. And then there's the bloodthirsty, murderous Death Knight, who turns out to be her long-lost half-brother Emile (currently under the identity of Jeritza), with the Death Knight being his Superpowered Evil Side that manifested when his father decided to marry Mercedes solely to have Crest-bearing children with her. Normally they fight for opposing sides, but in the Blue Lions route (the house Mercedes belongs to), upon completing the shared paralogue of Mercedes and Caspar you unlock an extra scene after the final defeat of the Death Knight where Mercedes comforts her dying brother and apologizes for being unable to save him in time. Meanwhile, on the Black Eagles route you can have the two reunited and fight together if you recruit Mercedes into your house, as her brother will be there after the time skip anyway. Notably, Emile/Jeritza is the rare example of a younger Monster Brother; also notably, he is a rather handsome fellow under all that scary armor.
- An animal example occurs in Pixie and Brutus in which the tiny sweet kitty Pixie meets the ex-military dog Brutus and recently adopted by Pixie's owner. Soon both developed a brother/sister relationship, being Brutus the Pixie's "big brother" and having all the instinct to protect the little kitty.
- A one-off comic by Pixie and Brutus creator Ben Hed has a normal-looking young woman approaching a Monsters Anonymous-type group, claiming to be a Non-Human Humanoid Hybrid of a minotaur and a mermaid. The werewolf in charge of the support group thinks she's mocking him and chews her out until she explains that she's just here to drop off her brother, a muscular mermaid-tailed bull man in a wheelchair.
"This guy buggin' you sis?"