In works of fiction characters will (most likely) have names. Most authors will go out of their way to avoid giving two characters the same name unless it's intentional for the sake of some kind of joke. This is exactly why it's extremely likely that If two characters have the same surname there is a good chance that they're related.
This could be played straight by giving two characters who are related to each other the same surname or it could be played with as someone could be a Long-Lost Relative or perhaps a secret partner. Sometimes one character may drop their spouse's name to reveal their maiden name is the same as someone else whose a main character in the series. Sometimes, if two creators accidentally use the same surname for their main characters, you can bet it will be mentioned by one of the other characters in the series at the very least if the two shows have a crossover episode - and the fanfiction writers will go mad.
Same Surname Fallacy: However, sometimes having the same surname does not signify the two characters being related. In the case of Loads and Loads of Characters, it could have been an honest mistake because the writer was not paying attention too closely. In other cases, the writer could have just been really lazy. These instances are not aversions of the trope, unless someone directly points it out and is immediately told it cannot possibly be for in-story reasons.
- In the Macross franchise, there's a United Nations Air Force airman named Edgar LaSalle who appeared in Macross Zero. In Super Dimension Fortress Macross, a United Nations Spacy officer named Claudia LaSalle appears in the show. Shoji Kawamori and the production crew have not cleared up any issues about whether the two are related. On the other hand, it's played with Sheryl Nome as the actual descendant of Sarah and Mao Nome. The connection was not seen when Macross Frontier aired on television. But a Blu-Ray release corrected this connection when Sheryl's earrings are seen near a picture frame of Mao's parents.
- Averted in UQ Holder!. Isana and Honoka Konoe briefly wonder if they're related to Touta upon seeing his last name (and take to calling him onii-sama), but there's nothing to suggest that there's any connection (especially because he's a hybrid clone of Negi and Asuna). That said, it's later turns out that they are related in a roundabout way due to the woman who gave birth to him as a surrogate also being a descendant of their grandmother Konoka (though the direct relation is not specified).
- Common in DC Comics.
- Larry Jordon, the Golden Age character Airwave, was later revealed to be a cousin to Hal Jordan, the Silver Age Green Lantern.
- Shierra Sanders, the Golden Age Hawkgirl was established in JSA as a cousin to the Golden Age adventurer Speed Saunders, even though the names weren't quite the same.
- A team-up between Superman and the Silent Knight had Clark taken aback by learning the Knight's real name was Brian Kent. The implication seemed to be that he must therefore be Pa Kent's ancestor.
- Conversely, Batman villain Harvey Kent had his surname retconned to Harvey Dent after his first few appearances to make it clear that he wasn't related to those Kents.
- When DC acquired the Quality Comics characters, Sandra Knight, Phantom Lady, became a cousin of Ted Knight, the original Starman.
- On that note, Phantom Lady is a notable Aversion. Despite three of the Phantom Ladies sharing a surname, none have been revealed to be related to each other.
- When she was introduced, Kate Kane, Batwoman, didn't appear to have any connection to the established Kanes in the Batman mythos. Her New 52 series established that Bette Kane, Flamebird, was her cousin, meaning she was also related to Kathy Kane (whose role as the original Batwoman had recently been reinserted into continuity). In Batman: Zero Year, Bruce's Uncle Philip has portraits of Kate's family in his office, suggesting Martha Kane's maiden name isn't a coincidence either. (Of course, in Real Life, they're all named after Batman creator Bob Kane.)
- In The League of Extraordinary Gentlemen (more specifically the prequel story "Alan and the Sundered Veil"), it is revealed that Randolph Carter is a great-nephew of John Carter.
- The Disney Comics originally introduced various characters by the name of "Duck" as unrelated to the main Donald Duck — just a case of Species Surname gone wild. However, as time went on, characters like Moby Duck, or, yes, his own girlfriend Daisy Duck, were retconned to be variously related cousins (not so much as a conscious decision as out of new writers assuming this trope was at play and that they always were meant to be related).
- In the Marvel Universe, Spider-Man 2099 debuted in 1992, introducing the world to Tyler Stone, the boss of Spider-Man in his civilian identity of Miguel O'Hara. A decade later, Tiberius Stone debuted in 2001 as The Rival of Tony Stark. No relationship between the two Stones existed until the 2013 Superior Spider-Man storyline "Necessary Evil", which established Tiberius as Tyler's father.
- Parodied and ultimately averted in the first comic crossover between X-Men and Star Trek in which both Hank "Beast" McCoy and Leonard "Bones" McCoy not only have the same last name, but they are also doctors. At one point, someone says, "Dr. McCoy" and they both answer at the same time.
- In The Uncanny X-Men and The New Teen Titans, one of Marvel & DC's early Intercontinuity Crossovers, during an Everyone Meets Everyone scene Changeling (Gar Logan) wonders if he and Wolverine (who at the time was known only as Logan when not using his codename) are related.
- Colossus from X-Men, real name Piotr Rasputin. Originally it was just a stereotypical Russian surname, but later it was revealed that the most well-known Rasputin was actually his ancestor.
- Sam Wilson, The Falcon, and Jim Wilson from 70s Incredible Hulk, began as a coincidence but were eventually retconned into uncle and nephew.
- Doctor Who
- The Doctor Who / Magician's House crossover "A Name of Ill-Omen" by John Elliott implies a connection between Stephen Tyler and Rose Tyler.
- The fanfic Osgood is written based on the premise that the three Whoniverse characters with that surname are different generations of the same family.
- In the This Time Round setting, the proprietor of Carter's Imports appears to be related to every fictional and real-life character with that surname.
- Panopticon Quest: The NWO renames its Operatives names that initialise to "JB", with certain surnames reflecting where one is in the hierarchy, meaning that Operatives sharing a surname need not be related. It is noted In-Universe that there have been people who made this mistake anyway.
- Subverted in Capitalism: A Love Story: Michael Moore interviews someone else by the same last name, and says "no relation" after giving his name.
- Parodied in Die Hard. When the two FBI agents arrive one of them says "I'm Agent Johnson. This is Special Agent Johnson...No relation." The joke is that one of the agents is black and one is white.
- Deliberately averted in Star Wars, where George Lucas intended for Antilles to be the galaxy's version of a common, generic last name like Smith. A New Hope has both Captain Antilles (of the Rebel blockade runner from the opening scene) and Wedge Antilles (the Ensemble Dark Horse Rebel pilot). The Phantom Menace also mentions Senator Bail Antilles. None of them are related.
- Emphatically averted by Bedlow Nobbs of Unseen Academicals, who is very much not related to Corporal "Nobby" Nobbs of the Watch, not even from a different branch but an entirely different family tree.
- In Lords and Ladies, Granny Weatherwax says she is distantly related to Arch-Chancellor Galder Weatherwax from The Light Fantastic.
"There was even a Weatherwax as Archchancellor, years ago," said Ridcully."So I understand. Distant cousin. Never knew him," said Granny.
- Averted and played with in Allen Steele's Coyote novels: In the first two novels of the series we have Captain Robert E. Lee (who traces ancestry to the old CSA general), who led the first colonial expedition to the planet and became an almost legendary figure in the planet's human history, and martyr in the resistance war with the Western Hemisphere Union colonists in the second book. Then in Coyote Horizon we have Sawyer Lee, a wilderness guide, who as far as he knows is no relation to the by-then late Captain Lee but always gets asked that (and notes that the surname coincidence probably helps boost his outfitting business a bit). By that time, there are a few hundred thousand people living on the planet.
- A Song of Ice and Fire:
- Justified in most cases by only the feudal nobility having surnames; two identical surnames really means at least some kind of relation. There are distant cadet branches whose relation to the main House no one remembers (such as the Lannisters of Lannisport), but they still are distant relatives and not just guys with the same last name.
- Averted with any nobleborn bastards, who are given a surname based on the region they're born in rather than inheriting their parent's name. For example, Jon and Ramsay Snow are unrelated; they just happen to both have grown up in the North.
- In Michael Moorcock works, any character with the surname "Beck", "Bek" or "Begg" is a member of the sprawling von Bek family who originally stem from the Ruritania of Mirenberg, exist across the entire multiverse, and in many male cases are alternate versions of Elric of Melnibone. In some later editions of his early work, Moorcock even retroactively changed character names to add them to the clan.
- Averted with lampshade hanging in Challenges of the Deeps, in which protagonist Ariane Austin is Earth's representative on an alien world. At one point, a minor character whose surname is also Austin gets himself into trouble by being a jerk to some aliens, and the alien leaders are worried this is going to cause a major diplomatic incident until they're assured that he's no relation.
- Angels Flight: The novel introduces two characters with the surname O'Connor. They aren't related.
- Played for extremely creepy drama on the Criminal Minds episode "Machismo": the B.A.U. agents and the Mexican police are trying to analyze the modus operandi of a Serial Killer that is annihilating old ladies and they discover that the old ladies share the same last names as the victims of some rape cold cases. Turns out that the man that is killing the old ladies is the rapist that assaulted the women before, and in a combination of devolution in modus operandi, wanting to show Who's Laughing Now? at some victims who mocked him, and plain old strong-arming the women to keep quiet, he's going after their mothers.
- In the Star Trek: The Next Generation episode "Booby Trap", the character of Leah Brahms was originally named "Navid Daystrom", and was intended to be a descendent of Dr. Richard Daystrom from the original series episode "The Ultimate Computer". However, Daystrom is black and the actress hired for the role was white (the casting department wasn't told what race she needed to be). Rather than keep the name and imply that she's an unrelated character who happens to have the same surname, they renamed the character.
- In the Blackadder II episode "Head" Ploppy is the jailer and Mrs. Ploppy is the woman who cooks Last Meals. They're unrelated.
Blackadder: You are clearly a woman of principle and compassion, mistress...?
Mrs Ploppy: Ploppy, Sir.
Blackadder: Ah, so you are married to...
Mrs. Ploppy: No, many people think that but it's pure coincidence. We did laugh when first we found out. "Good morning mistress Ploppy" he'd say, and I'd say "good morning Mr. Ploppy." [both laugh]
Blackadder: The long winter evenings must just fly by.
- Lampshaded on NCIS when Tony and Kate meet a doctor named Brad Pitt.
- Inverted (exaggeratedly) in the folk song "The Kellys", covered by Mick Moloney among others. The Narrator is seeking his uncle Kelly, but everyone he asks also happens to have that name, despite all being totally unrelated.
Well, I went and asked directions from a naturalized Chinese,But he says, "Please, excuse me, but me name it is Kell Lee."
- In Avenue Q, Princeton asks Kate Monster if she and Trekkie Monster are related, as they have the same surname. Kate finds this incredibly racist.
- Hamilton: In "Helpless", Angelica Schuyler introduces Hamilton to Eliza, who introduces herself as "Elizabeth Schuyler". Hamilton notes their common surname, and Angelica responds that Eliza is her sister.
Eliza: Elizabeth Schuyler. It's a pleasure to meet you.
Angelica: My sister.
- Averted so far in the Ace Combat Strangereal series, where at least three different women have shared both the given, and the family name "Kei Nagase": Phoenix's wingwoman in Ace Combat 2 (who also makes a brief cameo in Ace Combat 04: Shattered Skies), an Ouroboros member in Ace Combat 3: Electrosphere, and Blaze's wingwoman in Ace Combat 5: The Unsung War. No relation is currently known to exist between the three of them, except that the devs seem to love that name.
- When Kian's surname was revealed to be "Alvane" in Dreamfall: The Longest Journey, long-time series fans immediately picked up on his connection to Lady Alvane, The Narrator of the original The Longest Journey. The Wild Mass Guessing ranged from her being his long-lost mother (or aunt) to them getting married in the future. The mystery was officially settled ten years later in Dreamfall Chapters, where Saga gets Kian to adopt her, legally changes her surname to Alvane, and eventually becomes the Lady Alvane from the future whom April meets in the first game thanks to wacky time travel.
- Ace Attorney Investigations has attorney Calisto Yew, who has the same surname as the victim of a murder. After some initial teasing, she tells Edgeworth that they are sisters. This turns out to be a lie; "Calisto Yew" is an alias, and her real name is never revealed.
- The Hero of Three Faces
- Hermione Granger is the niece of Rev Geraldine Granger.
- Lois Lane and Margo Lane, which suggests some interesting things about heredity in their family.
- Spoofed in one comic in which the Doctor states that, in universes where Supernatural and M*A*S*H both exist, the Winchester brothers are related to Major Charles through their mother.
- Played with in Homestuck: The Big Bad of the story is the mysterious time-traveling Lord English. Many fans assumed he was either related to one of the heroes, Jake English, or even that he was Jake's future self. Turns out Lord English is Caliborn's future self. He took Jake's name as a Take That! gesture after his first defeat at the hands of Jake... and Jake inherited it from his grandma, who changed her surname to "English" in defiance to the Condesce, who feared no one but Lord English.
- Whateley Universe: Lupine's (Stella Woolfe) parents, with surnames of Wolf and Woolfe, were actually members of a long dispersed Native American clan, based around the wolf, and initially thought their similar surnames was just a funny coincidence. But, there are other characters with Wolf-related surnames, who are of unknown relatedness to Lupine, like Techwolf (Harry Wolfe).
- Farce of the Three Kingdoms: Liu Bei claims that all Lius belong to the royal family, especially him of course. This (usually) stops him from attacking other Lius openly, although backstabbing is still fair game.
- The Simpsons:
- In "Rosebud", a flashback shows that comedian George Burns is Mr. Burns' younger brother.
- Ralph Wiggum is referenced by the last name Wiggum a few episodes before he is canonically established as Police Chief Wiggum's son.
- In a throwaway gag in "The Blue and the Gray" episode, Lisa suggests buying a supermarket tabloid to see what "Cousin Jessica" is up to.
- In a brief cutaway in the "A Picture is Worth a 1,000 Bucks" episode of Family Guy, it's revealed that Kathy Griffin (an actress and comedian) is a distant relative of the family through Peter's side.
- Tiny Toon Adventures: Babs Bunny. Buster Bunny. No relation.
- Played with in the Kim Possible episode "Attack of the Killer Bebes". Dr. Drakken decides to avenge himself on the classmates who'd mocked him in college. It simply does not occur to him that the one surnamed "Possible" is in any way connected to his nemesis Kim Possible, and when he finds out that he's her father he rather absurdly insists that it's a common surname.