So, of course we won’t find a line in a comic that says “Rook Kast becomes the Armorer.”
Also, you are absolutely correct that the Armorer could turn out to be somebody entirely new or never get a name at all in the end (which would be kind of cool in the Spaghetti Western tradition to be honest).
However, my philosophy is that you should at least try and find the existing characters that might be connected to new media. Plus, there are some breadcrumbs that would lead you to believe the Armorer is Kast.
First of all, The Armorer seems to have deep animosity towards Bo-Katan. Rook Kast served in Death Watch under Pre Vizsla, who wielded the Darksaber. Maul showed up, killed Vizsla, and took the Darksaber, becoming the new leader of the Death Watch. Kast then became one of Maul’s most dedicated lieutenants, freeing him from capture by Dooku, fighting on Dathomir with him, and fighting against the Mandalore resistance at the Siege of Mandalore. So, this indicates to me that she follows the Darksaber, as won by Creed, as a matter of deeply held belief. Obviously, the Armorer also has those horns on her helmet, which may be a tribute to Maul.
Speaking of Death Watch, during the flashback to Din Djarin’s childhood, the Death Watch are who save Din from the droids and take him in as a Foundling. So, the Armorer’s covert seems to be an off-shoot or descendant from the Death Watch that saved Din. It would make sense why he defers to her judgment and guidance so much.
The Death Watch believed they were the true Mandalorian people, followers of the one true tradition:
“
We are the Death Watch, descendants of the true warrior faith all Mandalorians once knew. Now my people are living in exile because we will not abandon our heritage
” -Pre Vizsla.
Sound familiar to any dogma in the Mandalorian?
Long before the Clone Wars, after the Mandalorian Civil Wars, the Death Watch’s ancestors were exiled to the Moon of Concordia. During episode 5 of the Book of Boba Fett, the Armorer says:
“ Had our sect not been cloistered on the moon of Concordia, we would have not survived the Great Purge.” Concordia, although functionally a prison colony, is an important ancestral home for Death Watch.
Rook Kast was captured by Bo-Katan at the end of the siege of Mandalore and then we don’t hear from her again. If she escapes Republic imprisonment during the rise of the Empire, it would make sense she would escape to her ancestral home with the rest of Death Watch, Concordia. There again, if she was imprisoned on Mandalore, it would make sense that the new Mandalorian state exile Death Watch to Concordia like they did in the past for their crimes. Either scenario saves Death Watch from the imperial Purge on Mandalore.
She appears in Darth Maul Son of Dathomir and Clone Wars Season 7. That’s it.
So, while it’s possible they made up somebody else entirely-a brand new character. If we’re looking at existing characters that the Armorer may be, Rook Last makes the most sense.
An obvious flaw is how dedicated the Armorer is to not removing her helmet. My rebuttal to that would be that every Death Watch member removed their helmet during the Clone Wars. So, maybe in reaction to the devastating losses they took, the Armorer’s covert sank even deeper into the older tradition and faith of the Mandalorians, including keeping their helmets on.
