I guess this takes place before the word "guy" became ungendered

So characters also mis-gender Arty like me? Odd.

I also use guy as gendered... It became used differently when I was adult already.

Also, Artemius is a male name in our world (because it comes from gendered language, Latin, and before that from Greek; female equivalent if from Latin would be Artemia), so to be honest it's a bit jarring to me too (and I thought that she is male, for this reason).

Oh, time for some more lore into this fascinating and puzzling world.

For the gender sycophants in the comments: The comic likely hasn't conveyed it very well until now. I also thought Artemius was a guy before this moment since we can't hear them, since their choice of attire hides pretty much everything, and since it hasn't been explicitly mentioned until now. It's very likely she simply has a deeper voice that, combined with the above, confuses even the characters in this story.

As for the name, "Artemis" is the greek goddess of the hunt and is a female name in that language. "Artemius" seems to be a Latin/Greek conjugation of this same name for males, as indicated by Saint Artemius of Antioch - a male.

If nothing else, I'd say we were set up on this one, boys.

Artemius looks like a Mushi-Shi and is describing spirits like Mushi. The world thickens!

Eh, gender can be a delightfully complex thing. I mean, all we know is that Artemius, though she looks (and sounds?) like a guy, still prefers feminine pronouns (which really is everything we NEED to know)... But aside from that, she may well be an AMAB masc-presenting gender queer person with a preference for she/her pronouns or something like that. (Or, you know, she may just plain be a masculine-looking/sounding woman, no gender queer complexities involved at all.)