Character Overlap

Characters in one series Crossover to another series. A technique used to help define a 'Verse.

Anime and Manga

 * Characters from The Day of Revolution and Family Complex have roles of varying sizes in Princess Princess. All three are written by Mikiyo Tsuda.
 * Angel Links employees Valeria Vertone and Duuz Delax Rex appear in an episode of Outlaw Star. Similarly, Fred Luo, Clyde, and Iris all appear at various points in Angel Links.
 * Nasuverse. See below under Video Games.
 * Raildex.
 * Anything by CLAMP will have this. Tsubasa Reservoir Chronicle took this idea and went Up to Eleven
 * Prior to Tsubasa the title characters of CLAMP School Detectives were probably the worst offenders. Two of them were also the lead characters of separate series (though one only lasted a single issue) and the third was a supporting character in Duklyon: Clamp School Defenders. And this is before they put in a brief appearance in X1999.
 * Isaac and Miria, lovable robbers from Baccano!, make cameos as in Durarara!!. Since they're, having them be in Durarara's time period is easy.

Comics

 * Obscure 1980s black-and-white comic Tales from the Aniverse featured a cat bounty hunter named Miss Chevious. Cartoonist Randy Zimmerman carried the character over into her own series, which runs in Flint Comix & Entertianment over 20 years later.

Literature

 * Although the Discworld is technically one series, readers tend to group the books according to which recurring protagonist is the focus, and certain said protagonists cross over briefly into other books.
 * One of Bret Easton Ellis's signature techniques: Clay, the protagonist of Less Than Zero, is a minor character in The Rules of Attraction; Patrick Bateman, a minor character in The Rules of Attraction, is the protagonist of American Psycho, and so on.
 * Agatha Christie does this a fair bit. For example, Inspector Japp of the Poirot series appears in the first Tommy and Tuppence book.
 * Banjo Paterson's poem "The Man from Snowy River" namedrops the title character of another of his works, "Clancy of the Overflow." Clancy is also given a prominent role in the movie adaptation.
 * And "Harrison, who made his pile when Pardon won the cup" is presumably the narrator and placer of the victorious bet in Paterson's "Pardon, the Son of Reprieve".
 * Stephen King will often make brief references to characters and events from previous books, especially if those books took place in Castle Rock, Maine.
 * The character Hoid appears in all of the works of Brandon Sanderson set in the Cosmere (Mistborn, Elantris, The Stormlight Archive and Warbreaker) to serve various roles, such as a begger or a storyteller.

Live Action TV

 * Boston Public, Ally McBeal and The Practice had a crossover.
 * Alan Shore form The Practice went to join the law firm in Boston Legal.
 * Beverly Hills, 90210 begat Melrose Place which begat Models Inc. Characters like Jake from Melrose Place appeared on 90210 before Melrose Place began. Later Melrose Place characters appeared on Models Inc. (Amanda Woodward's mother and half-brother ran Models Inc.). Later, some of the original 90210 cast members returned for the reboot of 90210 and some of the original cast of Melrose Place returned for the reboot of that show as well.
 * Steve Urkel of Family Matters fame has appeared in both Full House and Step by Step.
 * The characters in Angel and Buffy the Vampire Slayer inhabit the same universe and cross-over regularly.
 * Rumor has it, if Firefly had continued, at some point, the crew would have been passing a bar, one of the characters in a long black coat would have said, "Nothing ever bloody changes," before turning around to reveal himself as Spike.
 * Thanks to a whole mess of anarchy, same thing went between Hercules: The Legendary Journeys and Xena: Warrior Princess. Considering that Xena was originally a villain character from Hercules who ended up getting her own show, it was frankly inevitable.
 * Characters from Cheers, including Norm, Cliff, Rebecca, and Frasier appeared on Wings, and later on the spin-off Frasier.
 * Carla, Norm and Cliff were also in an episode of St. Elsewhere titled, amusingly enough, "Cheers". Carla has also appeared on the short lived Cheers spin-off The Tortellis which revolved around her ex-husband Nick, their brood of children and Nick's new wife Loretta most of which were recurring characters on Cheers.
 * Dr. Roxanne Turner from St. Elsewhere showed up on an episode of Homicide: Life on the Street. Dr. Victor Ehrlich from St. Elsewhere appeared in the Homicide movie.
 * Richard Belzer has played Baltimore (later NY) PD detective John Munch on approximately a dozen different TV series.
 * In addition to Belzer's example, Law and Order SVU had Angie Harmon from the parent series consulting in the first series before they got their own, dedicated prosecutor. Who ever is playing District Attorney from the parent series will also occasionally have an appearance.
 * Characters from Stargate SG-1 and Stargate Atlantis regularly crossover.
 * Several characters from Stargate SG-1 have also appeared on Stargate Universe more than once.
 * McKay and Woolsey were both introduced as one-off antagonists in Stargate SG-1 before becoming regular characters on Stargate Atlantis. They both appeared in the Stargate Universe Season 2 episode "Seizure" to boot, making them the only Atlantis regulars bar Samantha Carter to appear in Universe.
 * Characters from Friends and Mad About You have crossed over. Mostly it's been Phoebe and Ursula Buffet, identical twins both played by Lisa Kudrow. Kudrow originally appeared as Ursula on Mad About You, and when she was cast as Phoebe on Friends, the two characters were made into twins to explain their identical appearance.
 * The Disney Channel did this with a triple crossover of some of their most popular shows, a mishmash called That's So Suite Life of Hannah Montana.
 * They performed another hodgepodge later, with Wizards on Deck With Hannah Montana.
 * Rare Panel Game example; before the final axing of its third part, Have I Got Buzzcocks All Over was an irregular Children in Need occurrence.
 * Jack Harkness debuted in Doctor Who then got spun off into his own show, Torchwood. Similarly, Sarah Jane Smith originated on Doctor Who back in the '70s and then returned in the mid '00s before receiving her own spin-off, The Sarah Jane Adventures, taking K-9 along with her. Both of them returned for the fourth series finale of Doctor Who which also brought along two other characters from their own shows (Mr. Smith and Luke from SJA and Gwen and Ianto from Torchwood). Earlier Martha Jones had appeared on Torchwood for a story arc and later the Tenth Doctor visited Sarah Jane on SJA. Both characters then later reappeared for the Tenth Doctor's last story along with several other characters who had appeared throughout his tenure. The Eleventh Doctor appeared on SJA bringing along Jo Grant who was a companion to the Third Doctor just before Sarah Jane.
 * In the Cold Open of the episode "Seminar" from The Office (US version), Michael Scott runs into David Brent, his counterpart from the original British series. David appears again in the season 7 finale "Search Committee" as one of the people auditioning to replace Michael as manager.
 * The character Director Leon Vance appears in both NCIS and, it's spin-off, NCIS: Los Angeles, being that he is the director of the Naval Criminal Investigative Service in CBS's NCIS franchise.
 * A very subtle version happens in on of the early seasons of NCIS when a minor character from JAG appears as to consult on one of her old cases.
 * The character Penelope Garcia is the technical analyst in both Criminal Minds and, it's spin-off, Criminal Minds Suspect Behavior.
 * The second season finale of Cougar Town features appearances by Ted Buckland and Abed.
 * The second season finale of 'Community' also features a brief blink and you miss it cameo of two characters from Cougar Town. Also Abed references his being an extra on Cougar Town in Community.
 * Naturally, the different iterations of Star Trek feature cameos from different characters, and share recurring nemeses like Q and The Borg. Worf is particularly notable for being a main character on both Star Trek: The Next Generation and Star Trek: Deep Space Nine. O'Brien also counts; he started out as a nameless extra in TNG's pilot episode, becoming one of the most recurring characters on the show before being pormoted to series regular on DSN.
 * Mr. Drucker appeared on both Green Acres and Petticoat Junction, as his grocery store was frequented by both show's characters. The two shows often had characters visit each other regularly in the early seasons.

Newspaper Comics

 * Bloom County cartoonist Berke Breathed carried over two characters from The Academia Waltz, a strip he did for his college's newspaper: Steve Dallas and Cutter John (then known as Saigon John).

Video Games

 * RPGs are well known for cameo characters making appearances in different games by the same developers, though these characters don't necessarily indicate that the games take place in the same universe, and are often a sort of internal Shout-Out.
 * Cless and Arche from Tales of Phantasia appear in Tales of Destiny, as does Clemente from Tales of Destiny.
 * Chelsea from Tales of Destiny appears in Tales of Eternia.
 * Tales of Hearts raises this to a gameplay element; scattered through the game are artifacts that let you summon characters from every prevous Tales (series) game to perform an attack for you in battle, from Arche to Ruca and Iria, and Caius and Rubia from the otherwise all but disowned Tales of the Tempest. As well as other characters from Bandai Namco games, like Gil, Klonoa, and KOS-MOS.
 * Gilgamesh from Final Fantasy V makes appearances in other Final Fantasy games, such as 8, 9, and 12. And, retroactively, I and VI.
 * Balthier from Final Fantasy XII and Luso from Final Fantasy Tactics A2 appear in the remake of Final Fantasy Tactics. Several FFXII characters also appear in A2, this time appearing to indicate that the two games do take place in the same universe. Same universe, same planet, even the same kingdom, for the most part. Ivalice, it's called. Vagrant Story's in there, too.
 * Dissidia Final Fantasy appears to indicate that all the FF games are part of the same "multiverse", if rather shakily.
 * Bleu/Deis appears in several Breath of Fire games.
 * Gabrielle Celeste and Ethereal Queen appear in all the Star Ocean and Valkyrie Profile games (made by tri-Ace). In addition, Lenneth and Freya from VP1 appear in Star Ocean 3; Puffy and Raddle the Traveler appear in all three Star Ocean games; Dirna Hamilton and Solon Solute from SO3 appear in VP2; Welch from SO3 appears in the remake of SO1.
 * Many of Nippon Ichi's games feature crossovers, such as Etna and Priere.
 * The Shin Megami Tensei series has included many crossovers, from the main character from Shin Megami Tensei: If... appearing in Persona and Persona 2, and more recently the Demi-Fiend in Raidou Kuzunoha 2. A debug of SMT3 revealed that Lisa Silverman was originally supposed to make an appearance, meaning that all the Megaten games could take place in similar universes. Or maybe not....
 * A handful of overlapping characters is all that holds the Type Moon multiverse together (the convoluted magic laws aside): Cool Old Guy Zelretch plays a large role in the backstory of Tsukihime and makes a cameo appearance in Fate Stay Night. Cool Big Sis Touko Aozaki is a major character in Kara no Kyoukai, is mentioned briefly in Fate, and her sister Aoko appears in Tsukihime and is the protagonist of Mahou Tsukai no Yoru.
 * While the older Ace Combat games were unambiguously set in the same Constructed World, their storylines were so self-contained that they could have just as well been taking place in parallel universes. However, a few overlapping characters linked the storylines together.
 * The Monsta and Mighta enemies from Bubble Bobble also appeared in the earlier Taito game Chack'n Pop.

Western Animation

 * Another Disney example -- Kim Possible, Ron Stoppable, and Rufus the naked mole rat teamed up with Lilo in an episode of Lilo & Stitch: The Series.
 * And again in another episode of the same show, this time with the titular character of American Dragon: Jake Long and his two best friends teaming up with Lilo and Stitch.
 * And double again with crossovers with Recess and The Proud Family.
 * One episode of the TV version of Disney's Hercules involved a crossover with the cast of Aladdin.
 * Many 1970's-era Hanna-Barbera adventure characters appeared in episodes of Scooby Doo, and vice versa, including: Jeannie, Speed Buggy, Dynomutt, and Josie and the Pussy Cats.
 * Similarly, a Space Ghost cliffhanger was resolved with the aid of the company's less-remembered Moby Dick interpretation.
 * And then there's The Jetsons Meet The Flintstones. Hanna-Barbera loved this trope.
 * DuckTales (1987) and Darkwing Duck have Launchpad McQuack in common as a main character, and Gizmo-Duck as a notable recurring one. Darkwing also saw the occasional cameo from Flintheart Glomgold, the Beagle Boys, and Magica De Spell.
 * One episode of the Ace Ventura Animated Adaptation featured Stanley Ipkiss from the animated adaptation of The Mask.