From The Heretic Knowledge Vault
"Guards" is obvious, but NPC stands for Non-Player Characters, a term brought over from unmentionable role-playing games. They're generally thugs or lackeys. They usually don't create much of a problem and almost always get blown up or beaten severely if any character of some importance has appeared on the scene. The term has roughly the same implications as a red-shirted ensign on Star Trek.
The Veracian Church seems to be the main employer of NPC Guards. The first time we see them, they're chasing an assassin and playing the role of city police, while the second time, they're guarding the Heretic Knowledge Vault, not very effectively. Since Veracia is a theocracy, the two roles are probably about the same, although there may be small differences in the cops' uniforms and the security guys'. Needless to say, it's not necessarily true that all of the NPC Guards appearances listed in the "Statistics" table involve the same hunks of electronic protoplasm; the whole point is that they're basically interchangeable or recyclable anyway.
The elves have NPC Guards too, as shown here and here. However, they're not interchangeable with the human ones. Even Tsuiraku and the Ensigerum find it useful to have some anonymous, disposable muscle around for guard duty, the former to watch over the Senilisized Meji Hinadori, the latter to keep tabs on Ian as he ponders whether to join Anita's crusade against the elves. These guards aren't interchangeable with the Veracian guards either. (Note, incidentally, that the Ensigerum and Tsuirakuan NPC Guards are coed, suggesting that they might be just a touch more socially advanced than the Veracian Church. Or not.)
Veracian guards' anonymity is assured by the fact that they all wear the same uniform of black trousers, coat, breastplate and neck-covering helmet, including silly epaulette things. This helps greatly in their NPC status, as from behind they have no distinguishing features whatsoever.
Most of them are so nondescript that they aren't even named in the strip, but there's one who's just barely important enough to have his name mentioned: Reeve. Sandel would fall into this category too, except that it's pretty clear that he's a wolf in sheep's clothing rather than a garden-variety guard. Tsuirakuan battlemages Jiro and Kenichi likewise have distinguishing characteristics (namely, that they're battlemages) and therefore get their own moment of glory, or at least, names.
Like most cannon fodder, NPC Guards are led by commanders who are just as expendable as the regulation guards are. An example (showing one getting expended, in fact) is here. The rank of this particular honcho isn't given, but the aforementioned Reeve was a sergeant, and higher ranks are known to exist; if Reeve hadn't been such a screwup, he might even have had one of those higher ranks. Interestingly, during Meji's encounter with what seems to be a Veracian police force, the one addressed as "Lieutenant" seems to be relatively low on the totem pole, in contrast to the real world where a lieutenant is an officer and a sergeant a lowly enlisted man (although usually one who knows more about the world than the lieutenant does). Is this just a quirk of the party that Meji incinerates, or a difference in the way ranks work in Veracia? Unknown.
Mortality among this crew is notoriously high, since after all, dying at the hands of more important characters is basically what they're there for, at least from a literary point of view. However, elven NPC Guard deaths reach a new high in the extended ambush and combat scenes of Chapters 33 and 34, where Ian takes out decades' worth of anger on them. See also Dinara, Galsu and Salaren, for a crew of elven NPC Guards who manage to acquire names before meeting whatever messy fate is in store for them.