Warning! The following contains spoilers for the Doctor Who Season 11 episode "Rosa." Read at your own risk!
Doctor Who revealed a new foe to Jodie Whittaker's Time Lord and her friends during the Season 11 episode "Rosa." Since then, fans have been trying to figure out the mysteries behind Krasko's identity. After all, it's often significant when the Doctor encounters another time traveler, so it stands to reason this villain is more important to Doctor Who than he initially seemed. That said, how significant can the new villain really be?
We're guessing he's pretty important indeed if he's linked to a particular phrase revealed in a previous Doctor Who episode. There's also the possibility that Krasko is someone important from the franchise's past that might not be recognizable to the Doctor (or fans) just yet. Here are our best theories regarding Krasko's potentially secret identity so far, as well as some evidence that could clue us all in on who he really is.
Krasko Is Captain Jack Harkness
John Barrowman's Captain Jack Harkness has not been seen on Doctor Who in a very long time, but it's possible the character resurfaced in last night's episode, only without Barrowman in the role. Krasko's vortex manipulator had many wondering if he was actually Torchwood's time-traveling lead. There were also the initials on Krasko's bag that read "G.F.B." Those could potentially be a reference to Jack's nickname, "The face of Boe."
If Krasko is Jack Harkness though, why doesn't The Doctor recognize him? Perhaps Josh Bowman is portraying a much younger Jack who has only just met The Doctor along his own timeline. That could explain the different looks and why neither one recognizes the other. If Krasko is revealed to be Captain Jack Harkness, he's got a long road to travel and some others things to work out, judging from his time in "Rosa." If Krasko isn't Jack, though, we're curious as to how he got ahold of that vortex manipulator.
Krasko Is The Time-Meddling Monk
Another wild theory sees Krasko's identity being tied to a Gallifreyan villain from the classic run of Doctor Who episodes. This theory identifies Krasko as Mortimus, or "The Monk," who went up against William Hartnell's First Doctor during The Time Meddler serial in Season 2. Like the Doctor or the Master, the Monk is also a Time Lord, although that phrase was not explicitly used in the series at the time of his debut. (He has since appeared in the expanded universe, as well.)
The Monk theory is rooted in Krasko's keen understanding of the TARDIS, and his desires to alter history in "Rosa." Time meddling has been the Monk's bag in Doctor Who lore, and while Krasko's motivations appeared to be more race-related, he could also just be wishing to create widespread chaos, much like the Monk. There's also the bit where Ryan possibly sent Krasko deep into the past, which could very well set the stage for the villain's Viking showdown with the First Doctor in 1066.
Krasko Is The Timeless Child
The Timeless Child was first introduced, so to speak, in Doctor Who's previous episode when the Remnants pulled the phrase from Thirteen's mind. The phrase mystified and confused the Doctor, and it's possibly tied to a repressed memory from long ago. It's certainly possible and relatively sensible for Krasko to tied into this mystery, especially given his first appearance happening in the very next episode.
That said, Krasko isn't exactly a child, timeless though he may be. The villain is rather young in his appearance, however, and when measured up against the age of the Doctor who is thousands of years old, a time-traveler in his 30s could be viewed as "a child." Krasko certainly makes his youthful mentality seem evident, so perhaps the "timeless child" phrase was meant to be more metaphorical than literal.