Batwoman has fixed a long-standing problem with secret identities in the Arrowverse by letting Mary Hamilton figure out that her step-sister, Kate Kane, is secretly Batwoman. There’s been a history in the Arrowverse of characters being kept in the dark way too long about secret identities, which didn’t reflect well on the characters. The plot-twist in Batwoman redeems the Arrowverse for this flaw.
In the Arrowverse, several key characters were oblivious to the obvious truth of a masked hero's secret identity until it was revealed to them. Iris West spent the majority of The Flash season 1 not realizing Barry was the Flash. She only found out when the Flash touched and shocked her late in The Flash season 1, causing her to remember when that happened with Barry while he was in a coma before becoming the Flash. Another example was Lena Luthor on Supergirl. Despite spending a lot of time with both Kara and Supergirl separately, Lena never figured out her secret identity. Instead, Lena learned it only when Lex Luthor told her that Kara was Supergirl, making Lena not look like the super-intelligent character she’s supposed to be.
On Batwoman, Mary figures out Kate's identity by putting a bunch of clues together, avoiding repeating the mistake made in The Flash and Supergirl. She hasn’t seen anything that confirms Kate’s secret vigilante life for sure yet, but the evidence pointing to it is incredibly strong. In episode 14, Mary pushed Kate to let her take an active role in the Duela Dent investigation from both the Kate and Batwoman angle. When Kate turns her down, Mary starts to lay on the guilt, saying that she accepts Kate no matter what and Kate can always be honest with her, almost turning antagonistic. This gives just as many storytelling opportunities as a more traditional secret identity story, without falling into the usual tropes.
Unlike other secret identity discovery plots, Mary has a lot to do here. She put the clues together: Nocturna’s attack on Batwoman, the ketamine Kate had in her system, Luke Fox being on an earpiece with Kate at odd times, and even Batwoman’s coming out interview. This isn’t an accident or being told the truth. Instead, Mary’s given an active role in that part of the story. Mary doesn’t stop at figuring out Batwoman’s identity, either. She will clearly go to great lengths to help others, as shown by the illegal clinic she runs. Of course, she’d want to help Batwoman with her work cleaning up the city.
This isn’t just a benefit to Mary’s character, but to Kate and Luke as well. Being a superhero or part of their support staff can often mean a character is bizarrely competent, especially around keeping secret identities. Mary is picking up on where they slip – Kate’s bad lie about the ketamine and Luke obviously using the earpiece in public. If Batwoman didn’t let Mary figure this out now, there’d have to be some sort of miraculous save to throw Mary off. A big theme to Batwoman is that both Kate and Luke are still learning how this works, so it makes more sense that they’d fail at keeping Kate’s identity that secret.
There’s still some more traditional secret identity superhero tropes going on in Batwoman, most notably the story with her ex-girlfriend Sophie. With Mary’s story, they’ve clearly learned from the other Arrowverse shows: it’s hard to keep a secret identity, and letting someone discover it and want to help can be good for just as much dramatic storytelling as the more traditional version of the story.