@MichaelCrank, tagging you to let you know this monster post is up
let’s do this. also as a note i’m not going to provide quotes for every single thing i’m referencing as this is already getting so fucking long but if anyone wants to know more about where specific things were mentioned just @ me or pm me and i’ll dig it up for you
Theories on M and their... situation ahoy, behind a cut cause a) long (oh god it's so long) and b) spoilers
Alright! So first things first, my running theory is that M is the result of experimentation done by the Agency. Perhaps to create stronger vampires (specifically for hunting rogue supernaturals), perhaps to emulate natural-born vampire characteristics in made vampires, perhaps for any host of other potential reasons. There’s simply not enough information to make an even slightly educated guess, and it’s also not too relevant to what I want to lay out here, which is simply that M is the result of experiments by the Agency.
I’m gonna start quoting things from here and doing it without spoiler tags because it’s behind a summary tag, so if you don’t wanna see spoilers here’s your last chance to back out. Also I’ll be pulling these quotes from the files because hunting for these in-game would take ages and I’m lazy, so excuse the code bits.
First clue, one SomethingSome brought up, is from one of Sera’s posts on tumblr, here. It’s about the cooking skills of Unit Bravo, and for M she mentions
M doesn’t cook-they’ve never needed to learn the skill what with having… other dietary needs… So any meal they attempted to cook might be, uh, hard to stomach, lol.
We know M used to be human, both because M themself tells us that F is the only natural-born vampire on the team (when you go check up on the thralls), and because they say it themself (though not where the detective can hear), just before you go to the hospital to investigate Murphy’s trashed lab.
“I may have been human once,” ${mason} adds, “but I still don’t understand them.”
(we get it M, you freaking edgelord, you’re ‘not like other humans’)
On a more serious note, this quote also points to their rather negative attitudes towards humanity, something they remark upon more often throughout the game. This could be simple cynicism (there’s plenty of humans who dislike humanity as a whole), but it could also be an artifact from being raised in the world of the supernatural since birth, and thus picking up some attitudes about humans that probably float around there.
Getting back to my point about cooking. We know M used to be human, and although we don’t know how aging works for turned vampires, I think it is a safe assumption that they either do not age or age very very slowly. The age ranges Sera gave here put M in their late 20s, which is an age where you would assume that anyone living a normal human life would have had to learn how to cook at least a little. This could imply that M never lived in an environment where they had to take care of themself, eg in an Agency facility and kept on a tight leash, or that even before the were turned something was done to them which led to them having “other dietary needs”.
Something else we know: M was chosen to be an agent and, in my opinion, to become a vampire in the first place. After you tell Kate about her son you can choose to question M on how the became an agent, and you will get this response:
I stare in disbelief over ${m_him} for a long moment. “How did you even manage to become an agent with your attitude?”
A sudden, strange smile flickers on ${m_his} lips. “Because they picked me specifically to become one.”
Again, this could be read as just M having been turned into a vampire, then recruited by the Agency because of their extraordinary skill, but I think it alludes to M either already being part of the Agency before being turned into a vampire, or being poached out of whatever their life was and turned because of some detectable skill/aptitude they had even when they were still human.
$!{m_his} words make me frown. “What the hell does the Agency do to you people?”
The question seems to sting ${m_him} and ${m_she} falls quiet, though ${m_she} doesn’t lose any of the stiffness in ${m_his} expression.
This exchange follows right after the last one. Whatever the Agency did to M, it has to have been bad for them to react like this.
In general, M doesn’t seem to be all that fond of the Agency. There were these two possible exchanges during the scene where you go check up on the thralls, which caught my eye as reinforcing that
boy i hope i can put a readmore within a readmore
“I bet the Agency likes to keep a lot of things that way…” I mutter.
${mason} snorts a tense laugh. “You have no idea.”
“I’m beginning to,” I reply, glancing over at ${m_him}.
“Yeah, I bet you are.” $!{m_she} thins ${m_his} lips. “The Agency would put secrecy before pretty much anything.”
The words sound less like a statement and more of a warning, but ${mason} turns ${m_his} focus back to the houses without saying anything more on it.
and
My brows pitch up a little, and I turn my focus back to the houses where the thralls are. “It would be fascinating to study how that works.”
${mason} shakes ${m_his} head. “Not exactly the word I’d use. Maybe you should huddle up with those techs at the labs and fulfil your needs there.”
There’s a strange tone to ${m_his} voice, even more so when ${m_she} chuckles at ${m_his} apparent joke.
I shake the thought away.
Whatever the Agency did to M, they’re (understandably) a little salty about it.
A general point: M is somewhat terrifyingly strong. This is emphasised several times, but it’s most clearly established during the attack on your apartment. Generally speaking your LI will be the one to go with you, but for the friend route it’s M that is sent to escort you out, as they are least affected by the DMB.
“But they’re alive.” Rebecca spares me only a glance before staring at ${mason}, her elegant features stiff and hard. “${nate} and I will hold the thralls back and try to help the others. You have to get my ${mc_son} out of here! You won’t be as effected by this as the others.”
Not just “you are stronger than the others so even with the DMB you’ll be best suited to keep them safe”, which is the reason behind M still having really heightened senses during the day, but “You won’t be as effected by this”. Later in the same scene, Murphy injects M with liquid DMB, and they are only knocked out for a little while. Compare this to Murphy, where liquid DMB is the nuclear option and the only hope while he’s in his super saiyan form. M isn’t immune to DMB by any means, but it sure as fuck sounds like they have some kind of resistance to it.
M’s weirdness is also emphasised when you are re-introduced to the team after the reveal.
“${mason}'s sensory and pheromone abilities are…much more powerful than most vampires,” Rebecca explains, though I note the hesitation in her voice—and also the frown that hits ${felix}'s face at the words.
I think we can all agree that that little moment of hesitation and F’s reaction are suspicious as fuck and Rebecca’s definitely hiding something from us here. I’m not sure whether she’s hiding something about what abilities M has other than pheromones and senses or if she’s skirting around the why or the how of m being so much more powerful, but there’s something she’s keeping quiet.
We know F isn’t great at keeping secrets and skirting around the truth, we see it more often throughout the books that someone (usually N) will spout some diplomatic answer or half-truth and F is visibly confused (mostly pre-reveal, of course). That could be what’s going on here, but I also think that there might be some bad blood (heh) from F to M about their abilities. Whether it’s because F’s natural-born and M’s a lab-made abomination or because power seems to mean status and thus privileges (which we know F really really wants) in the Agency and M is far stronger than F, there’s no way to know just yet. (i’d also like you all to know that i made a file with all the things i wanted to touch on in this post and this bullet point was written down as "M’s ‘skills’ 
". i feel like this is relevant information)
Another thing from the same re-introduction scene concerns M’s lack of surname. This scene is framed as full disclosure, no more secrets (though we already know that isn’t quite true when it comes to M) and yet M is once again introduced without a surname, and the only one with the rank of Specialist Agent.
Rebecca throws ${f_him} a glare, then moves towards ${mason}. “Specialist Agent ${mason}.” No surname follows.
The narration specifically notes the lack of surname, which could just be for readability but I’m more inclined to think that it’s there to highlight that hey. this is weird. pay attention. The reason I’m highlighting this specifically is because even F, who’s literally a being from another dimension, has a surname, either one they brought with them from the other side or one that they chose/which was given to them by the agency. Why doesn’t M? cause they’re a lab rat and property of the Agency is what i’m saying
Another comment that alludes to M’s past happens way earlier in the game, when you go to talk to the witness for Janet Greenland’s murder with M and N.
“It’s sad to think people have to live in these kinds of places,” ${nate} says, genuine sympathy in ${n_his} tone.
${mason} shakes ${m_his} head. “It’s not as bad as it could be. They should be thankful they have a place at all.”
We rarely see M empathise with or relate to anyone, let alone people they don’t know. To see them comment on this, to me, points to them having been in a situation where they didn’t have a place of their own. This matches up with the talk you can have with F later, at your apartment, about how they stay at the Agency but they don’t really have a place that’s theirs. F presumably did have such a place at some point, in the other dimension, and A and N have lived human lives as well, but I don’t think M has ever known anything outside the Agency. And to one who has always lived there, monitored and controlled and never truly their own person, a shitty apartment building like this would truly be something to be grateful for.
The conversation with F about how the Agency facility is not a home
“You don’t like the facility?”
“Sure I do,” ${f_she} says, chucking ${f_his} hat into the air and catching it, then repeating the action. “But it’s not exactly a home like this, you know?”
I’d never thought of it like that. “Do you guys always stay at the facility? You don’t have a place of your own?”
“Nope, no place that’s ours,” ${f_she} says. Though ${f_she} tries to keep it casual, a twinge of regret flashes across ${f_his} usually open expression. “But it’s not so bad!”
Then a detail that doesn’t so much point to M being an experiment necessarily (though I do choose to interpret it that way) as much as a shifty bastard with things to hide. M is very specifically mentioned to only ever wear long sleeves. The first time they are described it’s with long sleeves, though at that point it’s just another character detail. What made me take notice of this as something suspicious is when you talk to them in (front of) their room at the Agency facility (and they’re fresh out of the shower, way to come for my fucking life Sera).
${mason} stands in the doorway, hair slicked back and wet, droplets of water dripping from the ends. It dribbles lines down the small part of ${m_his} collar bone that’s on show before the rest is hidden by a buttoned-up black shirt. It’s the first time I’ve seen ${m_him} in anything other than a long-sleeved t-shirt—though this shirt is still long enough to cover ${m_his} arms to ${m_his} wrists.
That last line is what really caught my eye, and what convinced me that M has something on their arms/wrists that they don’t want people to see. Could be scars, though M doesn’t seem the type to care about people seeing scars they got in a fight, so they’d have to be something more sinister/something with more emotional value than just scars with no further story. Could be tattoos or other markings, but if they are deliberate markings I doubt they were something M did to themself as again, why would they hide them if it was their choice. I think whatever mars their arms that they don’t want people to see is a result of what the Agency did to them.
Last thing, which I don’t have any concrete thoughts about other than “this is odd and I feel it’s going to come back and be relevant, but I don’t know enough to make an educated guess about how or why”. The crystal. The very first time we meet M we notice their crystal necklace, which they hide as soon as they notice us looking at it. Now, M doesn’t strike me as the type to wear jewellery just for the aesthetic of it (although it is a v good aesthetic, gotta give 'm that), and if they were wearing it as jewellery, why would they hide it? I think this is either yet another thing the Agency forced upon them, or something very very personal (I only realised the second option as I was writing the next paragraph so I don’t have any arguments for or against it, just the thought)
The crystal is mentioned a second time in the same scene as the long sleeves, though this time M doesn’t feel the need to hide it from us (aww, see, they do care!). This is all supposition on my part and not really grounded in anything specific, but I believe the crystal is magical, and M wears it to help them keep their abilities in check. With the existence of supernaturals (especially fae) I don’t feel it’s a stretch to say that crystals can hold magical energies and be used for warding.
I think this covered everything @SomethingSome and I noticed and talked about, if anyone else has any other thoughts or noticed different things please let me know, i’d love to talk about it more!