What if Tomie is a whole species of self-replicating viruses, funghi or parasites: It can either infect female hosts and transforms them into a Tomie or it procreates by causing human males to become infatuated and chopping a Tomie up - causing them to grow once again. And eliminating other Tomies is at the same time another layer of evolutionary struggle, basically eliminating "less fit" Tomies from the gene pool. Even though the gene pool seems to be completely identical for every Tomie.
And they also seem to nevertheless have some sort of collective consciousness, as Tomies have access to memories from others, possibly without really knowing it?
Definitely fascinating thoughts for me. Wonder if any of these might turn out to be true in some way. And if it does, is Tomie genetically manufactured? Or does it exist naturally for millenia? Or does it even have supernatural origins? I am looking forward to the next chapters.