A number of characters have found their way into the main DCU continuity, including Cyclone, the female Judomaster, Lightning and Magog.
The KC version of Superman fell through a Negative Space Wedgie into the main universe for a while before returning home.
KC Superman, Batman and Wonder Woman once came to the past to meet their mainstream counterparts, while the KC Titans also met their present day DC predecessors. Now it's an alternate earth where the events happened concurrent to our present.
The KC version of the Joker's Daughter appears on Earth-16 in The Just #1.
Red Robin was a uniform and code name introduced on an older Dick Grayson in Kingdom Come. In the main continuity, Jason Todd briefly began using the name and suit in Countdown and shortly afterwards Tim Drake picked it up as his new permanent superhero identity.
Wally West's daughter Iris, the Kid Flash of the Kingdom Come universe, was brought into the mainstream canon during Geoff Johns' Flash run, albeit using the nickname "Irey" to differentiate her from her great aunt. However, as the DCU already had its own Kid Flash at the time (Bart Allen), Irey eventually adopted the costumed identity of Impulse instead. Eventually, when Wally travels to the future, he meets the adult version of Irey who sports the exact same costume as the KC version, just with long hair. KC Wally's other child, Barry, was also adapted, albeit with his name changed to Jai, with black hair rather than KC Barry's blonde, and Jai doesn't have superspeed.
Más y Menos were created for the show, and have since made appearances in the comics.
Cinderblock and Billy Numerous were created for the show before appearing in the comics. The 2023 Cyborg series from Dawn of DC would also bring See-More and Atlas into the DCU.
The cartoon's version of Gizmo was introduced in the comics as the son of the original Gizmo, similar to Ultimate Nick Fury making the move to the classic Marvel Universe as Nick Fury Jr./"Marcus Johnson".
The Starfire (2015) series brought Silkie (now called "Syl'Khee"), Starfire's pet worm from the cartoon, into the DC canon.
Cyborg (2023) #3 adds A.T.L.A.S. from the Teen Titans episode "Only Human" to the DCU. Interestingly, he's introduced as someone Cyborg has fought in the past, with his description of A.T.L.A.S.'s motivations suggesting their previous encounter wasn't totally dissimilar to the episode.
The aforementioned additions of Billy & See-More also add another immigrant of sorts. In that same issue, the Titans battle a version of Fearsome Five containing regular members Gizmo, Mammoth & Jinx, with Billy & See-More added in. In other words, this is technically the first official DC comics appearance of "the H.I.V.E Five," (who were based on the Fearsome Five) the Titans' rivals from the series.
Isis from the TV series The Secrets of Isis was introduced as an inhabitant of Captain Marvel's world of Earth-S, back in the days of the original multiverse. Decades later, the series 52 features a version of the character as a probationary member of the Marvel Family, though she meets her end in Week Forty-Four. She gets resurrected by Felix Faust in a later miniseries.
The Wonder Twins were brought into the DC Universe as members of the Justice League International spinoff group Extreme Justice, and subsequently became reserve members of Young Justice. The Wonder Twins finally received their first series in 2019, which also saw the official DCU debut of Gleek, their pet monkey from the cartoon.
Wendy and Marvin were introduced in The DCU in 2006 as caretakers of Titans Tower. However, they were super-genius, black-haired twins, so they had really little to share with the originals. In an ironic twist, they were viciously mauled by Wonder Dog, resulting in a dead Marvin and paraplegic Wendy. Although, to be precise, the dog they adopted turned out to be a shape-shifting monster controlled by the son of Ares. Oh, and their dad's the Calculator. Wendy later went on to appear as a major supporting character in Batgirl (2009), but was written out of the book just prior to its cancellation. After their comic book debut, the two were also brought over into the Young Justice (2010) animated series.
The Global Guardians originated in the Superfriendstie-in comic, before they made a proper in-continuity debut in a 1982 issue of DC Comics Presents.
Samurai, a Japanese superhero introduced in the same cartoon, would later be brought into the DCU during Brightest Day.
Minus the Gender Flip, it had happened before that in Justice League Unlimited, with the Ultimen. In fact, Young Justice's version of Apache Chief, a teenager of Apache descent named Tye Longshadow, even has the same voice actor as JLU's own Apache Chief pastiche... who was known as Long Shadow.
A one off villain from the 70s Superfriends comics named Kingslayer appeared in 2016's Superman #48.
El Dorado makes his first major canon DCU appearance in February 2017's "Suicide Squad Most Wanted: El Diablo and Amanda Waller # 5".
Jackson Hyde/Kaldur'ahm, the second Aqualad and son of Black Manta, was introduced during Brightest Day months before the Young Justice (2010) cartoon show he was created for debuted (thus making this case similar to the Barbara Gordon Batgirl, who was created for the 1960s Batman (1966) show but debuted in the comics first).
The sentient space cruiser Aya was introduced in the Green Lantern comics just prior to the New 52. Aya was created for the show, but like Batgirl and Aqualad, appeared in the comics first. She also showed up in the Smallville: Season 11 comics.
It took over 11 years since the series ended (and a detour to Young Justice (2010) and getting a Rule 63 version for The Multiversity) but Razer finally made his mainstream DCU debut in Green Lantern (2023) — and much like with Red X and A.T.L.A.S., it's implied that events similar to the show have happened as he's shown to have known Hal and Kilowog before his debut.
The Protector originated in the animated Teen Titans anti-drug PSA from The '80s, and has since appeared in a few cameos in the comics, including in Heroes in Crisiswhere he was one of the murdered heroes. He also showed up in Teen Titans Go! and Tiny Titans.
During the first season of the show, the comic relocated to World War II / Earth-2 to match and introduced the comics' version of General Blankenship, Diana and Steve's superior officer in the show.
Persephone, one of the Amazons from the Wonder Woman animated movie, appeared in several issues of Gail Simone's Wonder WomanVol 3 run. This was likely due to Simone having written an early draft of the film's script.
Similar to some of the MCU examples, Mystik U introduced a black version of Artemis who was modeled after the version seen in Wonder Woman (2017). The writer even says she's a big fan of Ann Wolfe, the actress who played Artemis in the movie.
Partial example: Tommy Merlyn was introduced to the New 52Green Arrow shortly after Arrow started. The timing and new first name can't be accidental, though the dark archer known only as Merlyn has long been a foe of GA. In the end, Arrow's Tommy Merlyn isn't the villain we know and love. His father, Malcolm Merlyn, is the Big Bad of season one.
Another partial example: Felicity Smoak already existed in the DC universe as a minor character, the stepmom of Firestorm. The New 52 introduces a new Felicity Smoak, one that looks almost exactly like Emily Bett Rickards.
Due to the fact that Shado had already been killed off by the time Emiko Queen was introduced into the show, the writers of Arrow created a new mother for Emiko named Kazumi Adachi. Teen Titans Special #1 subsequently confirmed that Kazumi Adachi was the name of Emiko's great-grandmother in the DC Universe.
Torpedo Man, The Claw and Magneto (no not the one from the X-Men) from the 60's Aquaman cartoon showed up in 1967's Aquaman #36.
DC Rebirth and several succeeding stories reveal that Watchmen's Doctor Manhattan has come to the main DC Universe. The rest of the Watchmen cast, such as Ozymandias and Comedian (and Rorschach, albeit as a Legacy Character due to the death of the original) follow in Doomsday Clock.
Back in 1941, The Adventures of Captain Marvel had Billy being given the power of Shazam! while on an expedition to Siam led by a John Malcom, and gave him a friend named Whitey. Malcom and Whitey were both introduced to the comics in Whiz Comics #22 the same year, and Whitey would become a recurring character. Interestingly, the comic story treats the serial as canon, at least in Broad Strokes, even though it not only reworks Cap's origin, but ends with the wizard taking the powers back because Billy no longer needs them!
The Black Manta miniseries brought Devil Ray, an expy of the title character created for Justice League Unlimited thanks to the Mercy Reef pilot, into the DC Universe, leading to Expy Coexistence.
The fifth issue of Static: Season One features the first appearance of Shiv, Talon and Onyx from Static Shock, as well as cartoon-based versions of Puff and D-Struct. The sixth issue also has a cameo appearance of Ebon. The African superhero Anansi would subsequently make his comic debut in a special one-shot in 2023.
The New Golden Age introduces John Henry, the hammer-wielding Black vigilante from DC: The New Frontier, into the history of the mainline DC Universe. Both his backstory and eventual murder at the hands of The Klan remain intact, but with the added wrinkle that the young boy seen at his tombstone at the end of the original story eventually went on to become a teen vigilante called John Henry Jr., who appears as one of the missing Legacy Characters in the subsequent Stargirl: The Lost Children mini-series, and is the great-uncle of John Henry Irons aka Steel.
The final issue of Dark Crisis introduces the Council of Light, the comics version of Young Justice (2010)'s primary antagonistic faction, the Light, right down to the initial depiction of the group as seven anonymous silhouettes of light.
The 2022 Blue Beetle: Graduation Day limited series introduced Victoria Kord from the live-action Blue Beetle movie into the official DC continuity. Like the Batgirl and Aqualad examples, this occurred before the release of the film due to the comparatively lengthy production cycle of most movies. The Green Beetle scarab from Young Justice also makes its DCU debut in the mini-series.
