Since Supergirl‘s first season, three things have remained consistently clear.
1) Cat Grant is the best unwitting mentor a superheroine could hope for, 2) Alex’s hairstyle ought to count as a superpower, and 3) Kara is terrible at keeping her secret.
This last item was even lampshaded in reference to the Martian Manhunter’s identity: for several episodes, only Alex was allowed to know who he was, explicitly because Kara couldn’t be trusted to keep her mouth shut.
Either because of Kara’s eagerness to have her friends beside her or because of the ridiculousness of Clark Kent-style glasses, quite a number of people have learned of her secret over the course of the show, and not all of them should.
Up to the third-season episode “Damage,” this is a compilation of people who know Supergirl’s identity, with an estimation of their respective risk of revealing it:
- Family. After the death of Kara’s aunt Astra, the remaining members of this group include Superman, Alex, Eliza, and Jeremiah. Of these only the latter is under suspicion, his whereabouts being unknown and his loyalties debatable at best.
- Close friends. Nobody in this group seems at risk of talking too much. Besides James Olsen, who learned Kara’s secret from Superman, this group includes Winn, J’onn, Mon’El and Cat (no longer being her boss, Cat definitely counts as a friend).
Exactly when she discovered Kara’s secret is not clear; it may have been at some unspecified point after J’onn posed as her in the first season, or it may be the case that Cat only pretended to be fooled by the impersonation and has kept this knowledge all along. This is an even likelier possibility if you remember how quickly she deduced who the Guardian and the Flash were. As for Mon’El, he’s presumably too busy staying alive to worry about revealing secrets.
- Friends of friends. This group includes Lucy Lane and Maggie Sawyer, and there’s a chance that even M’gann and Lyra know. From this list, the M’s are fully trustworthy, with the L’s being currently unseen and safely forgettable.
- Enemies. Oh boy. The list only keeps growing: Maxwell Lord, Lillian Luthor, childhood acquaintance Rick Malverne, religious fanatic Thomas Coville, and even the Music Meister and Mr. Mxyzptlk.
Lord is under blackmail from the DEO if he tells the secret, Mom Luthor is waiting for Lena to figure it out by herself and thus be heartbroken, Malverne had his memory deleted by J’onn, and Coville (allegedly) didn’t pay attention when Kara told him her name. As for the interdimensional villains, their intentions are anyone’s guess.
- Unknown numbers of random people. Kara has visited the alien bar in her civilian clothes, which wouldn’t normally arouse suspicion if she hadn’t openly ordered and drunk human-killing substances there.
Also, the Arrowverse heroes and their immediate associates are aware of Kara’s identity. It’s not impossible that Lillian Luthor and Rhea might have at some point mentioned Kara’s secret to, respectively, Cadmus members and Daxamite soldiers (several of which escaped to outer space to avoid the lead bomb).
We don’t know how many other plane passengers besides Coville might have caught a glimpse of Supergirl in the first episode.
And finally, the entire payroll of the DEO have seen Kara visit the monitoring room in both identities; considering all the times the DEO has been infiltrated, this is the set of people with the worst leak potential.
So, who should Kara keep an eye on? I’d be especially wary of Jeremiah, Lord, and above all Mom Luthor, who is on excellent speaking terms with her son and has every motivation to want to get revenge on Supergirl and, by extension, Superman.
But that doesn’t solve the biggest problem: Kara shouldn’t relax until J’onn sets up some method to perform 24/7 psychic supervision on every single extra who is visible in a DEO scene.
Seriously, who thought it was a good idea to let possibly hundreds of expertly trained and skilled employees witness one of the most dangerous secrets in the DC universe? Unless they’re being routinely mindwiped upon leaving for home every night, any of those nameless faces is merely an HR complaint away from instigating a worldwide catastrophe, and then only Rao will be able to save us.