Dev Diary #3 discusses Mercenaries and Artifacts:
http://muhagames.com/mom-dev-diary-3-september-2021/Mercenaries come in two types, Heroes and Champions. Heroes are more common, while Champions are more unique, plus have higher stats and more abilities.
One thing I like that Muha is doing with the remake is adding "extra", non-canon events, as some of these will unlock an extra choice/path if certain Mercenaries are present, while other events can result in certain Mercenaries joining you as a reward. It strikes me as a nice touch.
Artifacts can either be found or conjured, and your Mercenaries can be equipped with up to three of them at any given time. At the moment, there are no restrictions for building artifacts, although the devs are considering implementing one forbidding you from applying a spell/skill that is not from your wizard's magic school. (Which makes sense on paper, but I imagine time will tell whether this would be too restrictive or not.)
(Attached: Screenshots of a few of the Mercenaries from the original game, with their respective art upgrades.)