This one's my favorite so far, and it's not even close... or entirely because this one's not Ancient-adjacent. Zero's a fantastic choice for a focus character of a short story, being a character who rarely expresses herself, but does have a lot of history to work with, she's exactly the sort of focus character I get excited about seeing.
I love the approach to Zero's memory--it's something I've seen come up in other things, I think most prominently Doctor Who, that the human brain just isn't capable of keeping an immortal life's memories in it; eventually, something's gonna give. And it seems like for Zero, and potentially a lot of other voidsent, the way that manifests is just that they remember the emotional conclusion of their experience, but need to have their memory jogged about why they got there; in Zero's case, she remembers that she doesn't devour others, but took actively being asked to remember exactly why. Not entirely dissimilar form Elidibus.
I'm also naturally inclined to be interested in that story of Zeromus, too; myth and fairytales are famously a bit wobbly and inconsistent, so it might not necessarily be that the other voidsent told Zero the 'full story' of Zeromus; it's entirely possible that there's multiple versions of that story for different audiences and ends. Zero was told a story about being able to stand up for what's right, the other voidsent was told one about 'don't stand out from the crowd'. And that's not even getting into the question of if Zeromus was a figure rooted in fact or fiction.
Breaking some additional speculation into a separate spoiler box, because this starts to get a bit tangential:
If Zeromus' story is rooted in fact, as much as I loathe to admit it, it reads as a details-muddled version of the story of either Venat and her crew, or Azem; possibly both in that way mythology gets muddled by history. The 'darkness' in this case being Zodiark, and fighting for the light either being direct opposition, or assurance that there must be another option than mass sacrifice. The Zeromus story then gets right that they 'become pariahs', but miss the fact that it's voluntary... and crucially, seems to end right there, right when both Azem and Venat's crew also stop being easy public knowledge; surely if this were a mortal person who just got told 'get out of our city', there would be additional legends, but if it's a tale derived from either of those Ancient figures, it makes complete sense to suddenly get a 'they were never seen or heard from again'.