Though some of these questions might be answered in other Teen Titan universes, in terms of the canon of the series, they all are left a mystery. The movie Trouble In Tokyo resolved a few things after the cliffhanger finale of season 5, such as the romantic tension between Robin and Starfire, but it left much more unsaid. As in, Beast Boy never even mentions Terra.
The fandom can only speculate so much, and even after all these years, there has never been a concrete agreement on most of the secrets that the writers chose to withhold. And with hopes for an HBO Max revival far and distant, these mysteries will just have to stay that way, at least for now.
What Does Slade Look Like Without A Mask?
The barest shadow of his face was shown in "Apprentice Part II," exposing his short and spiky hair, but his facial features were never truly revealed despite his numerous appearances in the series. The reason for this mystery was probably to maintain Slade's menacing and enigmatic persona, but it was still crushing to the fandom that the creators never ended up unveiling his identity.
It's also a trait that Slade shares with Robin, as Robin also never takes his mask off in the show. This ties into the recurring theme of similarity between the two, and it gives more legitimacy to the fear that Robin expresses about one day turning into Slade.
Do Raven And Beast Boy Ever Get Together?
Although Teen Titans Go! certainly goes down this route, there was a much less explicit romantic connection between the two in the original Teen Titans. Beast Boy's main love interest was always shown to be Terra, and he and Raven only ever had a playful, somewhat antagonist relationship.
However, with all the romance surrounding them in other series and comic books, it's a legitimate question, and they certainly had potential. Beast Boy gave Raven the lucky 1-cent coin in "The End," and there were countless other times where the facade of their dislike faded to reveal the obvious care they had for each other.
Why Does Slade Hate The Teen Titans So much?
Slade randomly shows up in season 1 with the premise that he wanted to destroy the Teen Titans, and though this animosity lasted throughout the series, it was never explained in detail why he chose them in particular.
Perhaps it was just a geographic coincidence, as Slade enacted a lot of his villainy in the same region that the Titans protected, but it's still a real head-scratcher even so. Motivation is intrinsic to a good villain, and though Slade was definitely one of the best and most likable Teen Titans villains, giving him more specific motives would have added a lot of dimension and depth to his character.
Why Did Robin Leave Batman?
Robin only mentions Batman a few times in the series, and in the Teen Titan origin episode "Go," all he says on the matter of their separation was "Just moved here, and from now on I work alone." The tone suggests that they had a bad break, but as he later referred to Batman as his father in "The Apprentice," it's still unclear about the nature of their relationship.
Either way, these moments show what a profound impact Batman had on Robin's character, and it would have been such a treat to see the two physically interact within the show. Giving more background on his departure from Gotham would only have elevated the series, and it would have satiated a lot of the burning questions from the Teen Titan fan base.
What Does Robin Look Like Without A Mask?
The animators knew exactly what the viewers wanted in the only two instances where Robin is seen without a mask. His mask fell off in the episode "The Sum of His Parts" and the movie Trouble in Tokyo, and yet, they didn't really reveal anything. In both, his eyes were shown as simple black circles of surprise, and it was an intentional move by the animators to tease the fans who so desperately wanted to see his full face.
It's one of the largest questions of the series given Robin's icon status, but sadly, even if a sixth season was ever produced, this secret would probably never be revealed. It's just an ingrained quality of this character, who remains a fan-favorite version of Robin.
Where Is Blackfire?
Blackfire had her final appearance in the episode "Betrothed," and after Starfire bested her in battle and banished her, she menacingly said, "I won't forget this, sister dear!" Contrary to her words, apparently she did forget because she never came back to enact her revenge. And given that these types of lines usually foreshadow a return, it was a true surprise that Blackfire never made good on this promise.
It would have been a great callback had the creators included Blackfire in the season 5 villain arc, but because they didn't, no one knows what happened to Starfire's dark sister. It's a true mystery.
What Happened To Slade?
Slade was the only villain who had an arc that transcended multiple seasons, so it was a shock that he never reappeared after the epic season 4 finale when he teamed up with Robin. He was shown in a flashback in the episode "Things Change," which also featured a Slade robot replica, but his true character never showed up again.
It's such a burning question given that Slade was the most fleshed-out and important villain in Teen Titans, and it just didn't seem right that he would simply disappear. The build-up of Robin and Slade's antagonistic relationship was so intense that his character deserved more of an ending, and Robin certainly deserved answers to his questions about Slade's true intentions. As did the fanbase, really, who stood by in uncertainty all those years only to be left blind.
Who Is Red X?
Robin began the character of Red X in "Masks" to try to infiltrate Slade's lair, but someone new then took up the masked identity in "X," and it was never revealed who. It was implied that Red X would come back after his final appearance in "Revved Up," as he said to "Tell Robin we're even...for now," but he never arrived to make good on the "for now" part.
Red X's budding relationship with Robin was never fully explored, and he had a lot of potential to be an honorary Titan one day, which would have been an amazing redemption arc. It's a disappointment and a shame that his true persona was kept well-hidden, and it frustrates the fandom to this day.
What Happened To Terra?
After she was turned to stone in "Aftershock Part 2," Terra, one of the most powerful members of the Teen Titans, was mentioned multiple times throughout the series, as Beast Boy had fallen in love with her, and her sacrifice was a major part of the series. It was very out of the blue, then, when she came back to life in the finale "Things Change" with no premonition.
Her reappearance caused what should have been a satisfying end to Teen Titans to instead leave the fandom on a cliffhanger, and it certainly didn't help that Trouble in Tokyo never even bothered to sew up the mystery of her sudden resurrection. The creators tried to make it a given that Terra came to life, forgot her memory, and then just moved on, but no one is accepting that at face value. The fans really, really need an answer, and hopefully one day they will get it.
What Was In Robin's Briefcase?
The tension all throughout "Revved Up" was incredibly rich because of everyone's burning desire to see what was in Robin's briefcase, and this desire was equally shared by the viewers as well. It had such a build-up with all the villains suddenly appearing that it was truly a disappointment when its contents were never revealed.
Robin was also a very immaterialistic person, and his sudden desperation over this briefcase made whatever was inside even more enticing, making this mystery one of the most speculated out of the entire series. A major theory is that it contained a picture of his parents, which is a sweet idea, and fans should probably just settle with that.