(We're lumping these two together because they don't get a lot of screen time here, though we're hoping to see more of them in future entries.)
There are two kinds of bad guys in the Star Wars universe. There's the tall, scary, black-robed wizard types who throw lightning around with their hands and Force-choke underlings for failing to deliver their morning Danish at the appropriate temperature… and then there are the worker ants.
The worker ants are the regular military: the guys (and now, girls) who salute and follow orders. They seem to enjoy doing evil just as much as the big villains do. They're just much more…officious about it.
With their clipped British accents and pale, chinless faces, they're part of the enormous machine that keeps the Empire/First Order rolling ever forward to squash the galaxy flat.
These are the guys who have problems killing an entire planet but will get whipped into a frenzy if the paperwork is done wrong. The kinds of guys who won't give you a Band-Aid unless you've filled out the requisition forms in triplicate first. They're bureaucrats charged with executing the insane orders of the Sith in a practical fashion, and as such, they make evil look disturbingly like just another day at the office.
In A New Hope, it was Grand Moff Tarkin. Later, it was Admiral Piett and any of a dozen or so guys that Darth Vader Force-strangled on a whim. Here, it's General Hux and Captain Phasma: a strutting tin soldier and his silver-armored underling out to burn down orphanages, tie virgins to train tracks, and do whatever else Team Evil dictates.
Frankly speaking, they're pretty one-note characters. Kylo Ren struggles with his better half and sometimes lacks the courage of his dark side convictions. (Take careful note, folks: inner conflict = good dramatic material.)
Hux is just this strutting twerp, convinced of the righteousness of his cause and quivering his lower lip the moment anyone questions the fitness of his men…as Kylo Ren does at one point:
GENERAL HUX: My men are exceptionally trained, programmed from birth—
KYLO REN: Then they should have no problem retrieving the droid. Unharmed.
GENERAL HUX: Careful, Ren, that your "personal interests" not interfere with orders from Leader Snoke.
We can gather from that that he's not exactly best buds with Ren. But beyond that, he's all bloodless speeches and watery-eyed cruelty, the old Imperial Navy at its finest.
Phasma is even more of a cog in the machine. As someone farther down the totem pole, she has to get personally involved more often than Hux, actually shooting people instead of just casually ordering their executions over tea, for instance.
That makes her much more practical in her duties, concerned with getting the job done "right" rather than issuing the right commands. She takes Finn's defection kind of personally—look at how she recognizes him the moment he pops back up in her life after defecting:
FINN: Not anymore. The name's Finn, and I'm in charge. I'm in charge now, Phasma. I'm in charge.
But otherwise, again, she's about the armor more than the character (at least in this film).
Phasma is more interesting once you look beyond the character to the place she holds. In the first place, she's a woman—we literally haven't seen any female bad guys in the Imperial armed forces before—but also, we never actually see her face. She's wearing that silver helmet all the time, which actually echoes Darth Vader a little more than Kylo Ren does. Ren, after all, takes off his helmet to show us the deeply troubled young man underneath, but Phasma remains faceless and anonymous throughout.
That makes her a literal face of the machine: part of this huge First Order engine that grinds up people, planets, and eventually the whole galaxy if the good guys let it. Hux is cut from a similar cloth, and though we can actually see his face, he's still representative of a big, uncaring, anonymous engine.
They may not cut the scariest figures in the film, but the more you think about it, they're almost more evil than the Sith lords themselves: doing evil not because they're committed but because it simply doesn't bother them at all.