Transplant

When a character leaves one show to join the cast of another (often because the original show has ended). Basically a permanent Crossover.

Sometimes a character's Spin Off gets canned. When this happens, they'll often transplant back to their show of origin, with some flimsy explanation; if this happens, the chances of a Fully Absorbed Finale go way up. Compare Canon Immigrant.

Anime & Manga

 * Manga example: Subaru from Tokyo Babylon becoming part of the cast of X 1999. His Arch Enemy, Sakurazuka Seishirou, joins the cast as his opposite while his twin sister makes cameos.
 * Grown-up versions of the heroes from CLAMP School Detectives also appear in X 1999, as part of the Ancient Tradition supporting the heroes.
 * Katori Matou from Roman Club joins Amanchu! as scuba club advisor and Hikari's homeroom teacher.

Comics

 * At the end of Ultimate Power, an Intercontinuity Crossover between Ultimate Marvel and the Supreme Power universe, switched places. The latter is a major character in the post-Time Skip new Supreme Power series, while the former had some Destructo Nookie with the Hulk. After Ultimatum, Nick Fury crossed back over.
 * Ultimate Spider-Man also had its share of transplanting. First Kitty Pryde left the X-Men and became Peter's girlfriend for a while, and then they returned the favor when this universe's Liz Allan was revealed as a mutant and went to train at Xavier's. And after a Wham Miniseries and subsequent Time Skip, the Parkers have taken in refugees from canceled series: Human Torch and Iceman.
 * Peter David's Sachs and Violens, of the mini-series of the same name, were transplanted ten years later to Fallen Angel.
 * Charity, the onetime host of 70s DC "Weird" horror title Forbidden Tales Of Dark Mansion, later shows up in the 1990s Starman series.
 * The entire roster of Quality, Fawcett, and Charlton Comics superheroes were transplanted into the DC Universe after DC bought them all.
 * A few decades later, the heroes of Milestone Comics did the same thing.
 * And then the Archie/Red Circle superheroes and THUNDER Agents. Post-Flashpoint, Wildstorm is part of the main DCU (rather than Earth-50) as well.
 * The Sandman contains a number of horror host transplants: Cain and Abel are the most famous, but also Eve, the Three Witches, Destiny, Lucien and The Mad Mod Witch.

Literature

 * The kzin, from Larry Niven's Ring World, are essentially a whole race that's a transplant -- since their creation, they've showed up in Star Trek, Star Fleet, and even Dungeons & Dragons.
 * L Frank Baum wanted to move away from the Oz books and tried to start a new series with a new little girl protagonist, Trot. This series didn't do as well, and Trot ended up moving to Oz (in The Scarecrow of Oz) and getting immediately Demoted to Extra there.

Live Action TV

 * Kenneth the Page, the Ensemble Darkhorse of 30 Rock was originally a character on Late Night with Conan O Brien. Strangely even after the show was a success he continued to appear on Late Night and even stayed with Conan when he moved to The Tonight Show.
 * There were quite a few of these in Angel. In fact, in the latter half of the first season (after Doyle ), all three main characters were Transplants from Buffy the Vampire Slayer: Angel, Cordelia Chase, and Wesley Wyndam-Pryce. Then in the fifth season -- after Buffy had ended -- Spike was Transplanted to Angel as well. Also in the latter half of the fifth season, Harmony (a character who originated on Buffy and appeared in both series) was made a regular on Angel.
 * Detective Munch joining the cast of Law and Order: Special Victims Unit after Homicide Life On the Street went off the air. Law and Order: Special Victims Unit has another character, Captain Cragen, from the early seasons of Law and Order. Det. Lennie Briscoe went from Law and Order to Law and Order Trial By Jury, though only a few episodes were filmed with him prior to actor Jerry Orbach's death.
 * And Det. Mike Logan went from Law and Order to Law and Order Criminal Intent (after a telemovie).
 * Chief Miles O'Brien was a main character on Star Trek Deep Space Nine, having originated as a recurring guest character on Star Trek the Next Generation. Later, TNG main character Worf was brought onto the main cast in the fourth season of Deep Space Nine following the conclusion of TNG and release of The Movie Generations.
 * Next Gen recurring character Ensign Ro Laren was attempted to be Transplanted to a spinoff series twice, but each time actress Michelle Forbes refused due to reluctance to be tied to a six-year contract. Each time, the role planned to be Ro Laren was rewritten as an original character: for Deep Space Nine it became Major Kira Nerys, and for Star Trek Voyager it became Lieutenant B'Elanna Torres.
 * Again, this almost happened in Voyager, with Nick Locarno (a guest character from the TNG episode "The First Duty") being planned to be brought back as a main character. When it became apparent they wouldn't be able to bring back the character due to rights issues, they hired the guy who'd played Locarno, wrote for him a new character who was basically the same guy with a few details changed, and named him Tom Paris.
 * Enterprise-D crewman Reginald Barclay also popped up on Star Trek Voyager a few times.
 * A relatively unimportant and yet much-beloved recurring character on Stargate SG-1, Dr. Rodney McKay, crossed over to Stargate Atlantis. Oddly enough, this wasn't planned: the same actor auditioned for the part of "Doctor Benjamin Ingram", and he impressed the creators such that they decided to rewrite Ingram as McKay.
 * Followed by Samantha Carter after SG-1 was cancelled. Amanda Tapping was contracted for the never-produced season 11 of SG-1, so they brought her character to star in Atlantis for one season instead. After being in turn replaced as the leader of the Atlantis Expedition by IOA agent Richard Woolsey (yet another transplant from SG-1 who had recurring appearances in Atlantis before), Carter had a guest appearance in the Grand Finale, "Enemy at the Gate".
 * She wasn't intended to only be in one season, but Amanda Tapping wanted to leave to pursue Sanctuary so they had to replace her.
 * O'Neill has now been transplanted to Stargate Universe.
 * Deputy Enos Strait returned to The Dukes of Hazzard when his Atlanta-based spinoff ended after one season.
 * Similarly, Florence returned to The Jeffersons after her spin-off died a quick death.
 * Likewise, Joanie Loves Chachi's power-couple returned to Happy Days.
 * And so did The Lone Gunmen after their X-Files spinoff bombed... but only for one episode.
 * The popular couple Kevin and Lucy from the soap opera General Hospital was transplanted to to GH-spinoff Port Charles to attract GH's fanbase and anchor the new show to its parent show. When Port Charles was canceled six years later, Kevin and Lucy were nowhere to be found on the parent show despite maintaining plausible ties to the canvas. Then again, considering by that time GH was becoming a daytime romance novel version of The Sopranos and PC had become a daytime romance novel version of Buffy the Vampire Slayer with Lucy being revealed as a slayer and half the doctors (including one or two GH-legacy characters) becoming vampires, reintegrating the characters would have been tricky to say the least.
 * On the other hand, it was easier to transplant just about every character in the primetime spinoff Night Shift back to the mothership when it ended.
 * Some of them had never left GH in the first place; they were pulling double duty.
 * After The Golden Girls (and subsequently The Golden Palace) ended, Sophia joined Empty Nest.
 * After Civil Wars was cancelled, attorney Eli Levinson and his secretary Denise Ianello joined the firm of McKenzie-Brackman on LA Law.
 * Helen DuBois, who goes from Josh's boss from Drake and Josh to the principal of Hollywood Arts in Victorious.

Video Games

 * The main characters of the Fatal Fury and Art of Fighting series continued making appearances in The King of Fighters series years after both series' respective last installments: Garou: Mark of the Wolves and Path of the Warrior: Art of Fighting 3.
 * In a related example, Ikari Warriors Ralf Jones and Clark Still (also members of the massive KOF roster) returned to their run-and-gun roots in Metal Slug 6 and 7.
 * Ditto with the cast of Final Fight, who had an Intercontinuity Crossover with Street Fighter. Guy and Sodom crossed over into Street Fighter Alpha. Rolento followed suit in Alpha 2, and Cody joined the roster in Alpha 3. Hugo (who is heavily implied to be Andore) first appeared in Street Fighter III: 2nd Impact. Maki (from Final Fight 2) became an Ascended Extra in Capcom vs. SNK 2 before being added to updated releases of Alpha 3. Guy and Cody recently returned in Super Street Fighter IV, and Poison (after spending years as Hugo's manager) is playable in Street Fighter X Tekken. Other characters from the series (Haggar, Jessica, Carlos, Edi. E, the remainder of the Mad Gear Gang) make cameos around the aforementioned characters.
 * Speaking of Street Fighter, Blair Dame and Allen Snider from Street Fighter EX were bussed to Arika and Namco's fighting game, Fighting Layer.
 * Miku of the first Fatal Frame joined the cast of the third game.

Web Comics

 * After a couple of crossovers, Kestrel from webcomic Queen of Wands joined Something Positive when Queen of Wands finished, and still appears there. It's worth mentioning however that Something Positive fooled just about everyone into thinking this wouldn't happen, after they had Kestrel apparently crushed by a hit and run driver.
 * Explanation: The original appearance was a joke. Something Positive only got full rights shortly before Kestrel's 'real' appearance.
 * Gavin from Nukees joined Schlock Mercenary. En masse, as it happens.
 * The Midnight Crew in MS Paint Adventures first appeared as comical antagonists against Team Sleuth in non-canon, donation-driven strips. In Homestuck, all four members of the Midnight Crew appear to show up again, this time as real adversaries, with someone who eerily resembles Spades Slick acting as The Starscream. This fact is lampshaded in his formal introduction when the narrator tries to name him Spades Slick.

Western Animation

 * General Reginald Skarr from Evil Con Carne moved into the title characters' neighborhood on The Grim Adventures of Billy and Mandy after his show was canceled.
 * Mainly to serve as a joke character.
 * Launchpad McQuack was transplanted to Darkwing Duck (initially planned for Tale Spin) after Duck Tales ended. Gizmoduck also popped up from time to time.
 * A fairly obscure example is Goat of Megas XLR, who originated in a short-lived MTV show called Downtown. He's based on a friend of the creators, who also provides the character's voice and is occasionally credited as "Goat" or "Himself".
 * Goat later showed up on Metalocalypse, too. He sure gets around, doesn't he?
 * Figaro from Pinocchio has appeared in several Classic Disney Shorts and House of Mouse as Minnie Mouse's pet.
 * A cross-medium transplant occured from Squee to Invader Zim, the former being a comic and the latter being an animated show. Squee's teacher was named Ms. Bitters and became the teacher of Zim's class, and the horned aliens from issue one of Squee became the Vortian race and showed up much more frequently on Zim. Conversly, the wheelchair-bound aliens--which had shown up a good deal in Squee--only appear in one episode of Zim.