Jimmy Olsen, yes, that one, is going to take up superheroics on Supergirl season 2.

Here's one we never saw coming. Jimmy Olsen is going to become a vigilante on Supergirl season 2. More importantly, he's going to be using a superhero codename (and look) that will be familiar to DC Comics fans. His quest for superhero-dom will begin in the very next episode of the show.
"James is going to decide that he can no longer sit back and be a sidekick," Supergirl EP Andrew Kreisberg told Entertainment Weekly, "so he’s going to decide to become a vigilante and he’s going to become Guardian — complete with the shield — which is going to cause a massive problem in his relationship with Kara, because he has decided not to tell her."
Winn Schott will be James'"Felicity" providing technical support to The Guardian when he's out fighting crime at night. "Winn and James together is comedy gold," Kreisberg continued. "Watching Winn, as he takes his job at the DEO, but then he’s lying to everybody about how he’s spending his nights, because he’s going out with James, and how he walks in exhausted or walks in with a black eye and says he walked into a door, is the fun excitement of building these first 10 episodes.”
Now, we already met the comic book Guardian's alter ego, Jim Harper, on the first season of the show, so Jimmy is kind of taking his place. That version of Guardian was created by Joe Simon and Jack Kirby (you know, the guys who created Captain America) in 1942. Kirby resurrected Harper in the early '70s when he introduced another SupergirlTV show staple, Project Cadmus. We can't have these Cadmus storylines on this show without a Guardian running around, so here we go.

After the comic book Jim Harper version of Guardian, there were two others, and both were black characters: Teen Titans member Mal Duncan was the second Guardian, and then there was Jake Jordan, who fought in the pages of Grant Morrison and Cameron Stewart's absolutely brilliant The Manhattan Guardian comic, which was part of Morrison's Seven Soldiers of Victory line.
Now, hold your fire on this, because there's some precedent for this. First of all, Jimmy has taken up superheroics in the comics, albeit in the semi-humorous role of Elastic Lad during comics' Silver Age. We...ummmm...will probably never see that happen here. Unless, of course, they decide to tell us that James does have a past in Metropolis where that happened, in which case, I will follow this show to the ends of the Earth. And seriously, you can't have a guy with as athletic a build as Mechad Brooks on a superhero show and not have him kick a little ass every now and then.
The next episode of Supergirl, "Last Children of Krypton" airs on October 17th.