So we know a kernelsprite is really a combination of two parts, a kernel and a sprite, with the kernel being data (the process of “distilling the prototyped elements into something more symbolic, more essential”, according to Andrew’s book commentary) and the sprite being an organism made of ectoplasm, a ghost.
The Epilogues introduce us to the idea of a character’s “ascended essence”, which manifests as a flickering orb “like a little kernelsprite“. We’ve been over this bit already; the kernel and the “essence” are basically the same thing, the platonic “idea” of something, the data that defines what that thing is.
So given that in addition to having a central essence - of which there can only be one, since this essence is in fact the core of their Ultimate Self - a character can also have a multitude of “ghosts”, it would seem that the basic structure of the soul is basically the same as the structure of the kernelsprite.
The central core, or kernel, or Heart, is the static unchanging Platonic essence of the person, representing their nature. All iterations of a person share this single essence.
The ectoplasm, essentially the “flesh” of the soul, is a person’s Life: in the sense that the ectoplasm of sprites like Nanna is associated with the Life aspect and has healing properties, but also in the sense that this part of the soul actually represents a person’s life, their lived experiences, their lifetime, rather than just the idea they sprung from. Because one person can live any number of different Lives, they can have an infinite amount of ectoplasmas or ghosts; but these ghosts are just reflections or shadows of the core of the soul, the Ultimate essence.
The idea of ectoplasm as the shadow of someone’s true essence, a kind of waste product from the periphery of the soul, is reinforced by its presence in ectobiology; ectoplasm is what you get when you can’t appearify the real deal version of what you’re trying to appearify. This itself highlights the key difference between alchemy and ectobiology, which is that alchemy is a science of abstract ideas, combining truths into more complex truths, while ectobiology is a true physical science that deals with DNA, a fallible form of data that changes in various ways throughout one’s life and is only a slice of a reflection of what someone truly Is.