Spoilers below for anyone who hasn't watched tonight's episode of Supergirl.
Fans of The CW's DC TV-verse have seen an abundance of comic book characters grace their screens, with many more heroes and villains only getting referenced through visual or verbal clues. Supergirl, which spent its first season tiptoeing around the presence of Superman before introducing him fully in Season 2, dropped a huge easter egg in tonight's episode "The Darkest Place" that once again made it sound like Batman exists on this show's version of Earth. Here's how Kara put it in an early scene.
My cousin worked with a vigilante once. Lots of gadgets. Lots of demons.
The cousin she's referring to is obviously the Man of Steel, and the vigilante she speaks of is almost definitely Batman, although the creative team knows enough not to just plainly state game-changing things like that. The Caped Crusader definitely loves his gadgets, and he's faced demons both emotional and physical, so that fits, too. And this isn't the first time this season that a Bat-reference has come up, as one character namechecked Gotham City in a past episode. Even though none of this means Batman is coming to National City anytime soon, there's the small chance that he could, and regardless, there are still large implications that come from Batman being around in this section of DC TV.
Remember that Kara's Earth is not the same as those from other shows, as evidenced best by last year's Flash/Supergirl crossover episode. So if Batman is busting skulls and using his tech with Superman on that planet, does that mean he doesn't exist in the same dimension as Barry Allen, Oliver Queen and the Legends? We know from The Flash that the Scarlet Speedster is a part of the Justice League in the future, and what kind of a Justice League would exist without Batman?
Of course, it's just as possible that a Batman on Supergirl's Earth wouldn't be the vengeful creation of Bruce Wayne, but rather some other dark and brooding soul. That would allow for Bruce Wayne to still don his cape and cowl on Earth-1, although that's assuming that Wayne doesn't exist on that planet, with Earth-1 getting another vigilante taking the name. Or that there can't be more than one Batman. The multiverse is hard, guys.
Kara is sharing her distaste for vigilantes in reference to The Guardian, the on-the-rise hero that serves as James' secret identity, which makes it clear that there are, of course, a ton of other vigilantes in comic book history that Kara could be referring to. She's obviously not talking about anyone on Team Arrow, since she didn't meet them yet, but there are undoubtedly other shadowy do-gooders in this world using other kinds of boomerangs to stop flamboyant criminals. Superman probably knows all kinds of costumed weirdos. And we want to meet all of them.