Specifically, how Katja's whole "being in multiple places at once" thing works.
(There's a few oblique kink mentions in here, FYI.)
In short:
They can fork themself into multiple instances, which start out identical to the originating one, and which can (and eventually do) merge back into each other.
Nomenclature
While for basically all intents and purposes, they're just "Katja", I usually refer to them like this:
• Katja[0] is the "main" one, who is in charge of all of "Katja" generally's obligations — which involves handling far more than one person worth of responsibilities. This designator is more of a title that goes to whichever instance is currently "storing" "guests" and, say, helping people out with fursona TFs, which means they're also in charge of secretarial stuff for said guests, like "explaining" to concerned employers why they're nowhere to be found. [0] never ends up in positions where they cannot act on their responsibilities, instead forking instances who can experience those sorts of things while they keep being responsible. (They're also consistently dominant, socially and otherwise. They leave submissiveness for most of their other instances!)
• Katja[#] is any instance branched directly from [0].
• Katja[#][##] is branched from [#], so on.
How do they do this?
Well, their hard light, which also happens to be their soul, isn't really… matter. It has physical presence, but more in the sense of being a non-matter, non-energy fracture in reality that nonetheless has effects upon matter and energy. So it can be reshaped, merged, and split arbitrarily — and it can also be duplicated through what's basically budding.
In practice, this usually looks like Katja taking one of the hard light swords above their head and stabbing some convenient object with it, or just tossing it in the air in a safe direction. That sword will be replaced by another one above their head, and from the removed sword, their current configuration of hard light will appear, followed right after by another body. This new instance of them will be identical to the one who forked themself, with its consciousness branching directly from the originating instance rather than newly coming into existence.
How do instances interact?
Like separate people who just happen to be identical (or at least started out like such). They can do what might as well be telepathy via electronic means, but there's no magical pairing or bridging of any sort. Of course, having all the same memories and beliefs, it's not as if one will go rogue or anything.
What happens when originating instances change?
It doesn't change the branched instance. For example, there might still be branches kicking around somewhere from four ref sheets and a few genders ago, but they're basically certainly not ~in the wild~.
How do they merge back together?
The physical aspect of merging their bodies is variable (and if you know me, you can probably infer what it usually entails), but some things are consistent: their hard light merges back into one soul, and consequently, their consciousnesses merge back together. All the memories from each get re-synced into one whole, though the resulting instance will be aware of which branch any memory from that period came from.
Notably, this does not engage their "not dying" protection. It's a special case. Usually, irreversible merging would set that off, but here, each instance is deemed to be "them"; "the same person".
How many are there?
God knows, basically. It's rare that there's more than a few "outside" at any given time, but once you factor in the effects of parallel timelines and eldritch abominations, for example, you're looking at a number that's… …probably finite, or at least no more than countably infinite.
