Cartoon Network's original Teen Titans animated series was a very unique show. Between its anime-esque aesthetic, the multiple versions of its theme song and its emphasis on obscure comic book characters, the series was completely unlike anything else DC was doing at the time. One of the most interesting things that set Teen Titans apart from its peers was its approach to its characters' secret identities -- or rather, its lack thereof.
Series co-developer Glen Murakami wanted kids to be able to easily identify with Teen Titans' main heroes, and felt as though delving into their civilian alter egos would contradict this idea. "I thought that the minute you start giving them secret identities then kids couldn't project themselves onto the characters anymore," Murakami said in a 2004 interview. "And that was important to me."
As such, Teen Titans rarely explored the idea of its main characters having secret identities, and almost never used their real names. When the team went down to the local pizza parlor for a slice or to the video store to rent a movie, they did so in costume. In fact, they were even often depicted as sleeping in their costumes. They weren't Dick Grayson, Koriand'r, Victor Stone, Rachel Roth and Garfield Logan. They were simply Robin, Starfire, Cyborg, Raven and Beast Boy.
This rule was far laxer in the New Teen Titans spinoff and is virtually non-existent in Teen Titans Go!. That being said, even the original Teen Titans broke its "no secret identities" edict from time to time and used its characters' real names, be it subtly or overtly. In fact, by the time the show ended, the only main character to not have their secret identity referenced in some capacity was Raven -- though if you want to get technical, Raven is her real name and the "Rachel Roth" persona is a fabrication (not unlike Kal-El/Superman's Clark Kent identity).
Robin Faces Himself
While it's never explicitly stated, it is heavily implied that Teen Titans' version of Robin is, in fact, Dick Grayson. Perhaps the clearest proof of this is Season 2's premiere episode, "How Long Is Forever?," in which Starfire travels 20 years into the future and discovers that Robin has since taken on the Nightwing persona. However, while it's never spoken aloud, the name Dick Grayson can actually be seen later on in Season 2 during the episode "Fractured."
In "Fractured," Robin is visited by tiny, excitable version of himself from an alternate universe named Nosyarg Kcid. This, of course, is "Dick Grayson" spelled backwards -- and that name is clearly visible on-screen as Nosyarg introduces himself. The Titans quickly decide that "Nosyarg Kcid" is too difficult to pronounce and dub the new Robin "Larry."
Cyborg Goes Undercover
Despite it being uttered multiple times in Teen Titans Go!, the original Teen Titans never used Cyborg's full, real name of Victor Stone. That said, Cyborg did adopt the moniker of "Stone" to kick off Teen Titans Season 3's main story arc.
In the Season 3 premiere, "Deception," Cyborg goes undercover at the H.I.V.E. Academy as Stone, an aspiring supervillain with the ability to turn his body into solid rock. He is able to pull off this illusion with a pair of hologram rings given to him by Robin. But while "Stone" is able to gain the trust of his fellow H.I.V.E. students, headmaster Brother Blood ultimately sees through Cyborg's disguise.
The Wedding of Starfire
Like with Raven, Starfire's real name technically is Starfire, as that is the English translation of her Tamaranean name, Koriand'r. However, in Teen Titans, Starfire's teammates never refer to her as "Koriand'r" or "Kory," which they do in other pieces of DC media. Her home planet, on the other hand, is a different story.
In the Teen Titans Season 3 episode "Betrothed," Starfire is forced to return to Tamaran for an arranged marriage, which the team soon discovers was orchestrated by her villainous older sister, Blackfire. Upon Starfire's arrival, she engages in a playful screaming match with her adoptive father, Galfore, during which he refers to her as "Koriand'r." This name drop is somewhat subtle and rather easy to miss if you're not specifically listening for it, but it is definitely there.
Beast Boy and the Doom Patrol
Teen Titans' fifth and final season was a big one for Beast Boy, as it not only forced him into a leadership role for the first time, but also addressed his past as a member of the Doom Patrol.
In the two-part Season 5 premiere, "Homecoming," Doom Patrol leader Mento asks the changeling rejoin the team in order to take care of some unfinished business. Mento initially refuses help from the other Titans, though they follow along in secret and help the Doom Patrol out of a jam. When the two teams part ways in "Homecoming - Part 2," Elasti-Girl embraces Beast Boy and says, "We're all very proud of you, Garfield."
The other Titans (namely Cyborg and Raven) tease Beast Boy for his real name, implying that they'd never heard it before. Raven even quips that she's "gonna get a lot of mileage out of this one," though it never comes up again. That said, a later Teen Titans Season 5 episode does poke fun at the idea of Beast Boy having a secret identity in the first place.
In "Go!" -- a prequel episode that shows how the Titans first met -- Beast Boy is still wearing the mask he wore as a member of the Doom Patrol. His new friends suggest he get rid of it, though Beast Boy is reluctant at first, asking, "But what about my secret identity?" Raven responds, "What secret identity? You're green."