The writers at DC Comics have a certain penchant for dabbling in meta-fiction. This can all be traced back to the historic 1961 comic The Flash of Two Worlds, which was the first comic to establish there were two separate DC universes and that they were connected a DC comic book. But that's nothing compared to the fourth-wall shattering powers of certain DC characters.
There are several people in the DC universe who have come to realize that they themselves are living in a comic book. For most people this revelation would be an existential nightmare, but not for everyone. Here are all the characters in the DC universe who are lucky (or unlucky) enough to find out their whole world is nothing but ink on paper.
Animal Man
Beginning with the most obvious example, Animal Man was originally a D-list superhero without many fans in the real world. However, this all changed in 1988 when Grant Morrison's legendary run on the character redefined Animal Man forever by making him slowly become aware that he is, in fact, a D-list hero without many fans in the real world. That run ends in a spectacularly meta way when Animal Man meets Grant Morrison and complains about all the misfortune he had to endure in the comic for the sake of "drama." Fortunately for Animal Man, Grant Morrison is a benevolent god and decides to retcon everything bad that happened to Buddy Baker out of existence.
Ambush Bug
Ambush Bug is an interesting character to talk about, but the best way to think about him is DC Comics' answer to Deadpool (even though he came first). Just like Deadpool, this wise-cracking alien ant knows he's a comic book character and loves to joke about the absurdities of living in a superhero universe. Unlike Deadpool, however, Ambush Bug has teleportation powers which, combined with his meta knowledge about comic books, makes him one of the most chaotic "heroes" in all of DC.
Mr. Mxyzptlk
One of Superman's strongest antagonists with the hardest to pronounce name in all of comics, Mr. Mxyzptlk is a 5th-dimensional imp with reality bending superpowers that he frequently uses to prank Superman for his own amusement. Naturally, these powers make him aware of his own nature as a comic book character, and in Superman: American Alien, he decides to talk to the reader directly and prove that he's just as real as the person holding the comic book.
Lobo
The DC Universe's most infamous bounty hunter, Lobo isn't someone you'd normally associate with metafiction. But the Main Man started out as a parody of Marvel anti-heroes like Wolverine, so many of his comics are actually filled with meta-humor and the occasional fourth-wall break. Lobo only ever does this in his own books, however, and the best example comes from Lobo: Bounty Hunting for Fun and Profit where the Czarian mercenary tries to teach DC readers how to effectively become bounty hunters like him. He also gets one DC reader by the name of Leslie killed in the process, but that was probably an accident.
Etrigan
Similar to Lobo, Etrigan the Demon has been known to address his existence as a Comic Book character from time to time. And surprisingly for a demon from hell, he's pretty polite about it. In Garth Ennis and John McCrea's The Demon #58, Etrigan gives a special thank you to all the writers and artists who helped create him, including Jack Kirby, Alan Moore, and Alan Grant. And what's even more impressive is that he does it while rhyming!
Psycho-Pirate
Golden age villain Psycho-Pirate might have the worst luck of any supervillain. Being one of the only people to survive the original Crisis on Infinite Earths, Psycho-Pirate discovered during the event the existence of infinite alternate earths, including the real world where he's just a comic book character. Unfortunately for him, this knowledge drove him crazy and he ended up in an insane asylum. Psycho-Pirate would later return in Grant Morrison's Animal Man to finally address the fact that he knows the ultimate secret of the DC universe, and ever since he's recovered from his insanity and returned to being a supervillain.
Black Hand
If the name doesn't ring a bell, don't feel too bad. Black Hand is a very old Green Lantern villain who was only notable for his role in the 2009 Green Lantern event Blackest Night. But surprisingly enough Black Hand's debut issue Green Lantern #29 has a scene where Black Hand explains his evil plan to you, the reader. This is the only time where Black Hand has ever demonstrated this power, but it's also one of the oldest examples of a comic book character revealing they know they're in a comic.
Superman and Superman Prime
Part of the original structure of the DC multiverse is that every universe would have it's own version of Superman, including "our" universe. The Superman of the real world is Superboy prime, a DC comics fan who discovers he's actually the real Superman living parallel to the rest of the DC heroes. Like Psycho-Pirate, however, he was yet another victim of Crisis on Infinite Earths, and in the aftermath of the event he turned into the villainous Superman Prime and has been trying to retcon the DC universe back to it's original state ever sense. Superman Prime's rage at DC editorial would culminate in a scene from Adventure Comics #5 where Superman Prime tries to kill then Editor-in-Chief of DC Dan Didio.
But he's not the only Superman to become aware he's in a comic book! In Final Crisis (another Grant Morrison comic) The Man of Steel equips himself with the Superman Thought Robot-possibly the most powerful weapon in comics, ever. The Thought Robot is so powerful that it allows Superman to see outside the pages of the comic and recognize the reader. It's never specified if Superman remembers he's a comic book character after this event, but for a brief moment he was made aware he lives inside a work of fiction.
Grant Morrison
Now hold on, Grant Morrison isn't a character they're a writer! Well not exactly. Remember how Animal Man met Grant Morrison in the climax of their Animal Man run? A year after Morrison concluded Animal Man, Suicide Squad writer John Ostrander thought it would be funny to include Morrison (or "The Writer" as they're known in-universe) as a member of the Squad. Naturally the Writer's is fully aware they're living inside DC Comics and in true comedic fashion they're killed off immediately. Cause of death? Writer's block.