I got a good bit of constructive criticism regarding my first handful of entries into the A-to-Z Challenge, namely regarding a lack of background, especially for those unfamiliar with the game (which would be just about everybody). Some of that is just the pressure of cranking out all these NPC's, but I thought I would at least try to give some more insight into how I see these guys working in my own TUH games.
In TUH, humanity had superheroes and supervillains for over a century, but now there has been an explosion of spontaneously mutating humans manifesting powers. In addition, the discovery of alien life and cultures whose technologies far outpace what humanity had to offer has caused huge societal changes for the Earth. In less than fifty years humanity has gone from traveling through the solar system to spanning the galaxy, but their advancement has been under the very controlling gaze of the other races, especially the Xeros. Those same alien races have also been terraforming and colonizing planets and moons within our own solar system as well.
All of which I feel would predictably cause some pushback, anxiety about mainline humanity losing their place in a world now filled with different, more powerful beings. Ajax and his organization would represent one hostile response, a straight-up terrorist organization targeting aliens in the solar system, as well as notably mutants.
In TUH, there's a supernatural element to the universe, but it almost feels like it is the "alien's alien," one step further removed from the strangeness of the Kasen or the Xeros. I just finished the book Maplecroft, a horror novel that posits the historical figure Lizzie "Lisbeth" Borden as having killed her parents because of their being possessed by a Lovecraftian horror. Now living in seclusion with her sister Emma (who suffers from tuberculosis), Lizzie continues to seek out signs of nightmarish life in her small Massachusetts coastal town.
My Lizzie Borden is a vengeful spirit, a solitary figure on a crusade that appears to the outside world like the acts of a supernatural serial killer but in fact might mask a secret agenda that the PC's could discover if they were to dig deeper into the lives of her victims. I could foresee Borden becoming one of those gray NPC's who hop across the line of good and evil now and then, like the Punisher in season 2 of the Daredevil TV show.
I mentioned a little bit about mutates (TUH doesn't actually use "mutant" I suspect because it is so hackneyed in the Marvel canon). I see Cryo-Jenny as the slippery assassin, an ingenue who was get close to any public figure either through guile or charm or superhuman ability and then kill her victim with a signature move (freezing them to death). Cryo-Jenny would always be the facing issue of a bigger story, namely whoever was hiring her: organized crime, the Third World Union (a coalition of isolationist nations who embrace mutates), or another group.
Doc is a utility character, someone who can be more of a passive, non-combatant character for the PC's. He could be the origin of a host of monstrous villains, or could cause a headache by unleashing some weird alien tech on the world. In TUH there's a hint of a plotline regarding an heretofore unknown alien culture on Mars, and perhaps Doc could have come across something from that mysterious group. Naturally he would want to experiment with it, mostly likely by attaching to someone.