The protagonist of the first Tomie story apparently grew up with the series villain, Tomie, and was friends with her — and Tomie having a female friend is something completely unheard of in any of the other stories. In light of the much later chapters, this is only possible in two ways: that Tomie was a child "infected" with another Tomie, or that was the first Tomie and she started out as a normal teenager who was just murdered and dismembered by her classmates, and then discovered she could suddenly regenerate. In that case, whatever Tomie is now, she wasn't that way to start out with.
With how often and quickly Tomie creates a new copy, it is inevitable that Tomie will eventually become numerous enough to be noticed by the public. The question is, what will they do about her? Will they hunt down every single copy and educate the public on the danger of her, or will they become so smitten with her that a significant chunk of the male population will become her slaves?
Tomie seems to keep herself in check by ordering her thralls to eliminate her other incarnations. While Tomies seem to coexist while they are regenerating, we see that when Tomies who share the same territory meet and become aware of each other, they all instantly plan to eliminate their rivals. It might ironically be what's keeping humanity safe. But then again, maybe not.
Seeing as how Tomie only seems to regenerate after being injured, and even then the injury has to not heal for it to work, it's a little strange how Tomie can start regenerating from her head if she gets angry enough (first seen in "Photograph" and then much later in the series). After some thought, though, that Tomie might have indeed gotten injured—she might have had a (not so funny) brain aneurysm that erupted from stress.
Rereading "Painter", which also happened to be the point where Ito started getting serious about producing the series, suddenly jumped out with proof that Tomie is trying to get killed with clever manipulation beyond being a bitch. For one, Tomie's behavior changes at notable pauses, like she's analyzing the most effective way to act before doing it. When Mori finds her at Iwata's, she's silent and stares at him before rushing into a teary rant about nearly being killed, and since she was found there, Mori likely "rescued" her from this desired murder so she had to take him back. After, she seems to push Mori into murdering her, first by mentioning on two occasions that men try to kill her and specifically chop her up, which would cause replicatory regeneration, i.e. make more Tomies. This plants the idea in Mori's head, and Tomie tries to push him to the final act by criticizing and abusing him for his last, monstrous portrait of her. This doesn't work, so she pauses and instantly starts laughing instead, which she had seen upset him earlier, and he does just what she told him about, killing and cutting her up like she wanted. She abuses, but she also adapts and changes tactics, on top of planting ideas that will let her spread. This seems to be confirmed by Ito's author commentary, in which he said that the more committed approach to the series made him more intent on portraying Tomie's characteristics as clearly as possible.
A recurring piece of characterization is that Tomie’s favorite foods are caviar and foie gras. While it’s debatable whether she actually likes these foods, and it’s at least partially motivated by her greed and a desire to wind up her thralls by mocking them for not being able to provide such famously expensive delicacies, there’s also another dimension to it; expensive caviar is sourced from endangered fish, and its importation was outright banned in much of the world for many years, and foie gras is famously produced by force-feeding geese until their livers bloat and distend. In other words, these are both deeply, inherently cruel foods; and Tomie is nothing if not eager to delight in cruelty.
