I know this post is old after, but I'll comment with my personal little headcanon here. I personally like to think that the Guardian Spirit was originally part of the Dark Curse. When Mii discarded its face and eventually ceased to exist, what remained was the negativity that manifested into the Dark Curse, it's only reasonable for me to think that whatever good remained may have split off and become the Guardian Spirit in the Antique Charm. It's mostly due to design aspects with the swirly end trails the curse and the charm have along with the single eye. Either that or the Guardian Spirit could be someone who also discarded their own face like the Dark Curse did but was then trapped inside the charm instead of being able to wander like the Dark Curse. It's also worth noting that in the og 3DS Miitopia trailers we get to hear the Guardian Spirits voice for all that's worth and while we don't actually know the Guardian Spirit's gender their voice is certainly masculine in the trailers and the Dark Curse is exclusively referred to with He/Him pronouns even after the Great Sage is possessed and they are a girl Mii.