While the Marvel universe emerged more or less fully-formed in the 1960s, the evolution of the DC universe has been a more complicated process. Golden Age characters might have met up for Justice League missions but writers of individual books rarely cared about keeping continuity with other series, while the Silver Age introduced new versions of older heroes, the discrepancies in continuity explained away by the development of a multiverse. The expanding multiverse caused confusion, necessitating multiple event storylines over the years to reboot and condense the continuity, but this continual expansion and contraction of the multiverse has allowed the DC freedom not just to develop of its own characters, but to include other companies' superheroes into their own continuity.

RELATED: Killer Crises: The 15 Most BRUTAL Deaths In Every DC Crisis

The DC universe has hosted and acquired characters originally owned by Quality Comics, Fawcett Comics, Charlton Comics, and Archie Comics, as well as from formerly independent DC imprints such as Milestone and WildStorm. These introductions of familiar faces to a new setting have been happening since the 1950s. Some of these superheroes appeared briefly in the DC universe and then disappeared, while others have become major characters. Here are 15 of the most memorable superheroes who've switched universes when comic book worlds collide.

SPOILER WARNING for the general DC universe including the current "DC Rebirth" storylines.

15 PLASTIC MAN (1956)

Quality Comics was the first publisher to have its characters bought out by DC (called "National Periodical Publications" at the time), and the Quality Comics character to make the biggest mark on the DC universe has been Plastic Man. Created by Jack Cole in 1941, the mutated ex-con Patrick O'Brien's powers allow him to stretch to any form. The original Plastic Man comics were the most comedic of the Golden Age superheroes, and this quirkiness has earned the character big fans such as Grant Morrison, who made him a fixture of the Justice League in the '90s, and Frank Miller, who declared him the most powerful superhero in The Dark Knight Strikes Again.

National's purchase of Plastic Man somehow escaped the knowledge of editor Julius Schwartz, however, and in 1960 the suspiciously similar, more awkwardly named Elongated Man was created. Elongated Man's powers are more limited than Plastic Man's, and when the two characters meet up in comics and cartoons their rivalry is both tense and hilarious.

14 BLACKHAWK (1957)

Another Quality Comics character, Blackhawk was at one time the most popular comic character other than Superman, and the only character other than DC's Big Three to have his own series continously from the '40s through the '60s. The leader of an international squad of fighter pilots called the Blackhawks, the hero started off fighting the Axis Powers during World War II, but by the Silver Age (when the DC/National run began), his enemies were more of the science fictional variety.

Blackhawk's popularity faded in the late '60s, with Superman even calling him "washed up" in Blackhawk #228 in 1967. He and his team have occasionally pop up in comics, though, even to this today. An unrelated Blackhawks team had a short-lived eight-issue series at the launch of the New 52.

13 CAPTAIN MARVEL/SHAZAM (1972)

Here's one way to get access to another company's characters: sue them! DC/National put up a legal fight with Fawcett Comics for over a decade, with the fight being argued whether Captain Marvel was similar enough to Superman to count as copyright infringement. The case almost went to the Supreme Court before Fawcett settled and subsequently went bankrupt in 1953.

Captain Marvel and Superman clearly weren't that identical, though, as in 1972 DC started publishing new comics about the character. Ironically, copyright issues with Marvel Comics' own Captain Marvel character now prevented them from titling the new series "Captain Marvel," instead calling it Shazam, after the magic word young Billy Batson says to transform into the superhero. After 1985's Crisis on Infinite Earths event, Captain Marvel joined the main DC universe. As of 2011, he's officially been named Shazam to avoid any confusion with DC's main competitor.

12 UNCLE SAM AND THE FREEDOM FIGHTERS (1973)

Every comics publisher in the early '40s needed its own patriotic superhero. DC/National's Wonder Woman wore red, white, and blue to fight Nazis, Marvel/Timely's Captain America punched Hitler in the face, and Quality decided to turn Uncle Sam himself into a comic book hero. Sam made his first DC Comics appearance in Justice League of America #107 as the leader of the Freedom Fighters, a team consisting of other obscure Quality Comics characters such as Doll Man, Black Condor, Human Bomb, Phantom Lady, and The Ray.

Spectre #37-38 elaborated on Sam's mythology, making him the literal Spirit of America created by the Founding Fathers and given strength by people's faith in America. A 1997 Vertigo miniseries by Steve Darnell and Alex Ross had the character facing darker points in American history.

11 BLUE BEETLE (1985)

In 1983, DC acquired the rights to the Charlton Comics library of characters. They asked an up-and-coming Alan Moore to write a new story featuring them. Moore's comic left most of the characters unusable, so they changed the Charlton characters into original characters and that story became Watchmen. The actual Charlton Comics characters first made their DC debut in Crisis on Infinite Earths, where their universe got folded into the main DC universe.

Blue Beetle, the inspiration for Watchmen's Nite Owl, has actually gone through three publishers and three incarnations. The original series was published by Fox Comics from 1939-1950. The powers of the first Blue Beetle, Dan Garrett, fluctuated a lot over this run, going from a powerless crime-solver to using vitamins for strength to possessing a "sacred scarab." Charlton bought the character but introduced their own Blue Beetle, Ted Cord, in 1964. This was the character DC continued to write until Infinite Crisis #3 in 2006 when they introduced Jamie Reyes, the new teenage Blue Beetle with an alien powersuit.

10 CAPTAIN ATOM (1985)

DC was quick to create its own version of Captain Atom, the inspiration for Watchmen's Doctor Manhattan, after acquiring the character from Charlton Comics. The original Silver Age version, created by Joe Allen and Steve Ditko, was a scientist named Allen Adam who gained atomic powers in a rocket accident. Allen hasn't been seen in the comics since DC Comics Presents #90 in 1986. The DC legacy character, Nathaniel Adam, is an Air Force pilot who survived an experiment during the Vietnam War and was sent forward in time.

Distant from humanity, Captain Atom has become a villain multiple times, absorbing Kryptonite to become Kryptonite Man in Superman/Batman and being turned into the new Monarch in Infinite Crisis #7. He was also flung into the WildStorm universe, back when that was separate from the main DC universe, in the nine-part limited series Captain Atom: Armageddon.

9 THE QUESTION (1985)

The Question was one of Steve Ditko's creations after splitting with Stan Lee. He was a superhero based on Ayn Rand's Objectivist philosophy that was deconstructed through the character of Rorschach in Watchmen. The post-Crisis DC version of the character, defined by his 1987-90 solo series by Dennis O'Neil, drew significantly from Zen Buddhism and other Eastern philosophies. Rick Veitch's 2005 series added a more psychedelic edge to the character, redefining him as an urban shaman.

The original Question, journalist Vic Sage, trained Gotham ex-cop Renee Montoya to become his successor in the series 52 before dying of lung cancer off-panel following issue #38. In the New 52, Montoya's role as The Question was retconned, but multiple different versions of The Question were featured, including two different ones on the New 52's main Earth as well as one on Earth-4, who is closer to Ditko's original characterization.

8 THE SPIRIT (2007)

The Spirit was one of the first creator-owned comics. Graphic novel innovator Will Eisner wrote and drew almost all of the pulp adventures of detective Denny Colt since 1940 with the exception of a few early stories when Eisner was drafted into World War II. The stories were published originally in newspapers, then by a variety of comics publishers over the decades. In 1996, Eisner allowed a number of other artists and writers to do their own takes on the character for Kitchen Sink Press, so precedent was set for DC to hire Jeph Loeb and Darwyn Cooke to start a new Spirit comic in 2007, two years after Eisner's death.

This was not just the launch of a new series, but the incorporation of The Spirit into the DC universe. In DC's first new Spirit story, he met Batman. Batman/The Spirit won the 2007 Eisner Award (an award named after The Spirit's creator) for Best Single Issue. The Spirit hasn't stayed tied down to DC, though; IDW and Dynamite have since licensed the character for comics, including in a Rocketeer crossover.

7 HARDWARE (2009)

Milestone Comics was an imprint published by DC created as a collaboration between Dwayne McDuffie, Denys Cowan, Michael Davis, and Derek T Dingle. The group of African-American writers and artists created their own shared universe with an emphasis on superheroes of color. Hardware, created by McDuffie and Cowan, was the group's longest running series, lasting 50 issues from 1992 until 1997, when Milestone stopped publishing comics. The hero, Curt Metcalf, is a genius inventor whose boss is secretly his nemesis.

Hardware briefly entered the DC universe as part of the "Worlds Collide" DC/Milestone crossover in 1994, but he and the other Milestone heroes entered the DC universe full-time due to an interdimensional rift created in the aftermath of Darkseid's death in Final Crisis. In the New 52, he became a mentor figure to Static. In 2015, it was announced the Milestone characters would take more prominent roles in a series of "Earth-M" graphic novels, though these are still in the works.

6 ICON (2009)

Icon, which followed an alien stranded on Earth working as a lawyer while working with his energy-manipulating human sidekick Rocket, was one of Milestone's flagship series. By far the most conservative of the Milestone heroes, Icon is a favorite of Supreme Court Justice Clarence Thomas, a fact which made Dwayne McDuffie feel uncomfortable writing the character.

In the DC universe, Icon became friends with Superman and a member of the Shadow Cabinet, a superhero team consisting of other Milestone heroes. His appearances in the comics have been sporadic, but on the animated series Young Justice, he served as a member of the Justice League, while Rocket joined the main team midway through the first season in the episode "Usual Suspects."

5 THE RED CIRCLE HEROES (2009)

Not every attempt to bring other companies' superheroes into the DC universe has been successful. DC had big plans when they licensed the characters from Archie Comics' short-lived Red Circle superhero line. DC had licensed the characters for their self-contained Impact Comics line that barely lasted two years from 1991-93, but now they were going to be sharing space with the likes of Batman and Superman.

J. Michael Straczynski was tapped to reintroduce the firestarter Inferno, the immortal Hangman, the patriot Shield, and the technologically-enhanced Web to readers in a series of one-shots in October 2009. Solo titles for The Shield and The Web lasted ten months, then the team comic Mighty Crusaders crashed and burned after just six. DC gave up the rights to the characters after that. Archie has since relaunched the Red Circle heroes under their new Dark Circle Comics lines.

4 STATIC (2009)

That Static officially joined the DC Comics Universe so recently might come as a surprise to those familiar with him mainly through the 2000-2004 animated series Static Shock, in which Static was part of the Bruce Timm DC Animated Universe (creator Dwayne McDuffie also wrote for the Justice League cartoon). But while electric-powered teen Virgil Hawkins was hanging with the Justice League on TV, he was notably absent from comic stores. Due to Milestone's demise, only a single four-issue miniseries, "Static Shock: The Rebirth of Cool," was released during the show's run.

Static is easily the most popular of the Milestone characters that are now part of the DC Universe. He's fought enemies such as the Terror Titans and Holocaust, and in Teen Titans #69, he officially became a member of the Titans. As part of the New 52 relaunch, he got a new series of his own by Scott McDaniel and John Rozum, though it only lasted eight issues.

3 STORMWATCH (2011)

WildStorm was originally Jim Lee's imprint at Image Comics in 1992, publishing both stories in a main shared universe and various creator-owned titles. It got bought out by DC in 1998 but everything was business as usual for the WildStorm series until the imprint got shut down in 2010 (the imprint was revived this year with Warren Ellis' new The Wild Storm). In the big New 52 relaunch of 2011, the WildStorm universe characters officially joined the DCU. A new Stormwatch series featured a team of Apollo, Midnighter, Jack Hawksmore, The Engineer, and Jenny Quantum and ran for 30 issues.

The breakout characters of the Stormwatch team have easily been Apollo and Midnighter, a sort of reimagining of Superman and Batman as a married couple. Midnighter got his own 12-issue series in 2015, the first time a gay male character had been the solo focus of any Big Two comic. Following that series' cancelation, a Midnighter and Apollo six-issue miniseries was released as part of DC Rebirth.

2 WILDCATS (2011)

Unlike "Stormwatch," Wildcats has not yet been brought back as its own series following the merger of the DC and WildStorm universes. Multiple characters, however, have been featured in DC comics. Both the psychic special forces operative Grifter and the Human-Daemonite hybrid Voodoo got solo series as part of the New 52. Voodoo by Ron Marz lasted 12 issues, while Grifter went through two writers (first Nathan Edmunson, then Rob Liefeld) before ending with issue 16.

Other Wildcats characters have shown up in other DC New 52 series. The shapeshifting martial artist Warblade was killed by Deathstroke in Ravagers #1. The Kherubim warrior Zealot made her first DC appearance in Deathstroke #9. One of the Wildcats' enemies, the Daemonite commander Helspont, fought Superman himself in Superman #7 and #8.

1 THE WATCHMEN (2016)

The most controversial group of characters to be incorporated into the DC universe is the most recent one. In a move that surely has Alan Moore doing his best to summon his snake god Glycon to place a curse on all involved, the DC Rebirth is hinging on a crossover between the traditional DC heroes and the Watchmen. DC Universe: Rebirth #1 revealed that the entire New 52 continuity had been the creation of (presumably) Doctor Manhattan. The Button had Batman and The Flash investigating The Comedian's button.

In November 2017, a new storyline, "The Doomsday Clock," will continue the crossover between the DC and Watchmen characters, focusing on Superman and Doctor Manhattan. Whether you love or hate this idea, there is some rich thematic material that can be mined out of the meeting between an alien dedicated to protecting humanity and a man who's grown alien to his own kind.

Do you know of any other characters who made the jump to the DC universe proper? Let us know in the comments!