*had been waiting for this!*
View attachment 361437
*came prepared!*
Notty, I have a glass of theory for you.... so I'll make a list of clues...
1) Thea uses medical procedures to change body, thus a Doctor would be the best candidate, since she/he could access the files of every person on the planet, their medical files and records (those will be important for 2)
2) before she assumes an identity she (because her latest body was female) needs to learn that person's history and behaviour patters, this takes time but if she hadn't made her presence known it can only mean she's preparing
3) even if she studies a person, there's one problem... people have dark secrets and obviously there can be someone knowing about thenm so unless she knows even the most hidden part, she could be demasked if for example the commander has a MLP secret fetish and Thea doesn't know about it, a brony greating the commander with their secret handshake could discover it's a body double. This is where a truth inducing drug would come in handy....
4) Thea wants to destroy the system; what better way than to make the elites stop lying? That's obviously a gamble on the debauchery and corruption of those people... to get to them, the commander is the best candidate, who would suspect a hero with medals dingling on his uniform... as the commander Thea could slip the drug into anyone's drink and get them talking... but let's give her a flaw... she's impatient and the procedure takes time... she waited for years and now she would have to wait few more hours? No way... it would be better to find someone who looks quite like the commander and just tweek him
5) Marcus shots blanks... with Trisha in the bedroom it can't be a hardware issue, nor the software... because that woman... she could arise the dead! so there is another explanation.... one thing science is unable to re-create are working sperm and blood, let's assume that part didn't change in the future and speculate that Thea simply doesn't care about that feature since she only thinks about her mission...
putting all pieces together Marcus is Thea