This image was made by an artist called Nix. It also features a character named Nix. I can tag the character, but I can't tag the artist. If I add the tags artist:nix character:nix, the artist will not be saved.

How do I properly tag the artist?

I've tried finding other works of this artist on e6 to discover alternative names, but I haven't seen any (although it's possible that I overlooked one).

Single tag can only be one single type, just a reminder that in these situations NEVER change tag type if it's already in use for something else! Always check if there's stuff under the tag you are changing and you can also change the tagtype from Tags on top panel to avoid this from happening.

Also in that case, fluff-kevlar is second artist.

Well, I suck at reading. In any case, this would mean that it's required to move all stuff under nix -> nix_(artist) and nix_(nix) and probably alias nix -> nix_(disambiguation) thanks to this:
https://e621.net/tag?name=nix*&type=&order=count

A tag can only have one type. If an artist and a character share their name you may have to use name_(artist) and name_(character).