The following contains spoilers from Superman #5, on sale now from DC Comics.
The latest run of Superman has provided the Man of Steel with a new status quo, but one of the biggest mysteries facing him is the addition of Metropolis' latest hero: Marilyn Moonlight. Readers know very little about her beyond hints her deep connection to Metropolis' past. However, Superman #5 (by Joshua Williamson, Jamal Campbell, and Ariana Maher) brings the character back into focus, and the manner in which she appeared has troubling implications.
Following the fight with Silver Banshee, Clark and Lois were on their way to a double date night with Jimmy Olsen and Silver Banshee. However, as the couple walked through the streets, Marilyn Moonlight watched them from on high, seemingly focused on them specifically. Considering that she wouldn't have many reasons to watch them, there is a chance that Marilyn may know Clark Kent is Superman.
Marilyn Moonlight Might Know One of the World's Greatest Secrets
Now, Marilyn Moonlight has only been appeared twice thus far, and both events did little to explain who she is, let alone her motivations. However, both her appearances were within the same vicinity as Superman. The first instance was during Parasite's rampage throughout the city when Superman checked on the Daily Planet. He found Marilyn there, and after a cryptic warning, she vanished. Now here she is again, watching Superman's alter ego with interest.
It's not a lot to go on, and though "twice is coincidence," considering that it's Superman, the implication here is that she knows who he really is. It would certainly add to the unknown factor she represents in the current conflict taking over Metropolis. Neither Superman, nor his new enemies, Doctors Pharm and Graft seem to have much knowledge about the unknown heroine, and if she knows who Superman truly is, then she has suddenly become a much more dangerous player in the game depending on how she uses that information.
Marilyn Moonlight Might Be Watching Someone Else
Of course, there is another possibility: Marilyn might not be spying on Superman, but Lois Lane. Marilyn's first appearance was in the Daily Planet during an attack that affected Lois. Adding to this, Marilyn seemed to be there before Superman, perhaps having had her own reasons to check up on the Planet. It would also explain why she was watching the Kents at the end of Superman #5. She wasn't spying on Superman, she was spying on his wife.
Lois has stated that she would begin investigating the origins of Marilyn Moonlight, and considering she is an expert detective, that would give anyone pause. It's possible Marilyn is keeping an eye on Lois because doing so would attract unwanted attention. If so, she genuinely doesn't know that Superman and Clark Kent are one and the same, and is likely acting as something of a guardian angel to Lois. If she does know who Superman is though, then he will need to learn as much about her as soon as possible to even the playing field.