Now that Electra is out of the Daredevil picture and the Punisher has his own show, Season 3 of the Netflix series is taking it back to basics. The focus is on Matt Murdock, his nemesis Wilson Fisk, and everyone else in the Hero of Hell's Kitchen's circle — most of whom, by now, know exactly who that hero really is or was before he was presumed dead at the end of Defenders. When did Matt Murdock reveal his secret identity to Karen on Daredevil? She's known the truth for a while, but they still have unfinished business.
Prior to the third season of Daredevil, Matt Murdock joined the other street level New York heroes in The Defenders. Karen Page appeared in both The Defenders and The Punisher after that — when Matt was supposedly gone for good. At that point, however, the cat was already out of the bag. Matt told Karen he's Daredevil in the Season 2 finale.
After telling her, when fans met back up with Karen and Matt in The Defenders, they weren't... not... broken up? The relationship status was difficult to pin down, understandably, but seemingly ended sometime during the crossover. Despite writing sympathetic op-eds for the New York Bulletin about vigilantism in the city, Karen attempted to distance herself from both Matt and Daredevil.
Why is this relevant in Season 3? Rumors surrounding this darker, surly season of Daredevil suggest that the series is adapting the "Born Again" comic book storyline written by the notoriously twisted Frank Miller. In that arc, Karen is a drug addict and leaks Daredevil's true identity to pay for her next hit. While Netflix's investigative journalist version of Karen doesn't seem likely to go down that particular path, it is possible that Matt could be grabbling with his vigilante secret going public in Season 3. Whether or not Karen has anything to do with it, knowing who knows what and when could prove important.
One of the nice things about the Marvel Cinematic Universe, a precedent set by Iron Man right off the bat, is that the characters don't tend to have secret identities. These heroes have emotional baggage, that's for sure, but keeping secrets from loved ones isn't even in the overhead compartment. The exceptions to this rule are few and far between, but include characters like Peter Parker and, of course, Matt Murdock.
The tropes that Karen Page has to suffer through as a love interest unaware of who she's actually involved with are well-worn in comic movies and shows in general but rare in Marvel properties. Thankfully, she wasn't kept out of the dark for too long. Did Matt break up with her to "protect" her, as so many heroes do? Yes, but at least he wasn't additionally lying to her. Plus, Karen has at least one secret of her own — remember how she killed James Wesley in Season 1? She hasn't told anybody about that.
Since The Defenders happened pretty soon after the end of Daredevil Season 2 when Matt came clean to Karen about who he is, and Matt vanished at the end of The Defenders, the two of them haven't really had the time to figure things out. Hopefully Karen and Matt will get some closure about his secret life in Daredevil Season 3 — before some kind of tragedy inevitably strikes.