WMG / Shiki

Sunako was mind-controlling Seishin

.Thus explaining why he so readily did everything in his power to serve her, and sheltered her by the end of things.

It's chance who comes back as a Shiki and who doesn't...

...However, it's not wholly random chance. There's probably some baseline probability that people come back as Shiki after being killed by Shiki, no matter their circumstances (thus explaining why innocents like Toru and Ritsuko could become vampires). However, certain things about an individual—which could range from genetics to state of mind—can boost that chance of coming back as a Shiki to well above the baseline, which is why so many of the Shiki we meet are strange or troubled in some way and were when alive.

• It's confirmed later in the series that it's something genetic. If one person rises as a Shiki, it's very likely that other people related to them directly by blood will also rise. However, this is still not guaranteed...

Jinrou are not true vampires. Rather, they are human beings that can activate and deactivate their vampire attributes in a number of ways.

This is chiefly based on the notion that, as Tatsumi put it, Jinrou “never died” but rather transformed at the brink of death. Their superhuman qualities are all totally dependent on how much human blood they consume.

The sole exception to this might be their basic longevity, as while Tatsumi explains that Jinrou require sufficient blood consumption to access their full capabilities, he does not explain whether or not they need blood to cease aging.

The times we see Jinrou manifesting their Shiki qualities, it is during times of emotional upheaval (such as when Tatsumi is angered by Natsuno’s feigned ignorance during their first confrontation), feeding or imminent feeding (Tatsumi’s fangs being visible when he attacks the Ozaki clinic, and Yoshie’s fangs and Shiki eyes when feeding on fleeing villagers), or seemingly at will (Seishin’s eyes changing when trying to convince Sunako to leave the village with him, and Tatsumi’s right eye changing after accepting Tohru’s first invitation to his house).

These things come together to suggest that rather than becoming a subspecies of Shiki, some humans merely gain the ability to activate superhuman qualities in exchange for new blood.

If this theory is valid, it would be more correct to refer to Jinrou as “werevampires” or “wereshiki” (translated into Japanese, of course) rather than “werewolves.” Some even theorize that the Shiki are controlled entirely by their blood (with the insinuation being that vampirism, in this universe, is a curse of blood rather than body and soul).

It would make sense that in the event of the blood curse failing to entirely dominate a person’s system- even if the struggle persists until near death- he or she would instead assimilate the curse and become able to manifest its characteristics with new blood being the reagent.

Jinrou, at least in the anime and manga, are those who willingly gave their blood to vampires from the start.

We see this happening with Seishin, and at least in the anime and manga, it's true of Natsuno as well. We don't know enough about Tatsumi or Yoshie to know how they originally became Jinrou, but that means there's nothing to say that their situations were any different.

• Given how Tatsumi willingly defers to Sunako, he may have done the same as Seishin did.
• This troper thinks it's more or less with the fact that they weren't sucked consecutively in 3 days. Thus allowing their living bodies to adjust longer and become Jinrou.

Masao's Missing Mom died in childbirth.

She's curiously absent from the family and nobody ever speaks of her, suggesting that she's been dead for a really long time. Given how much older Masao's brother is than him, and how old his father is in general, his mother would also have been pregnant at an older age, and older women are more likely to die in childbirth than younger women. We also know that medical care around Sotoba isn't airtight, because Ikumi had children who died in infancy.

Sunako is not originally from Japan, or if she is, she's not as old as she's commonly made out to be.

In the flashback we see of her as a normal girl, she's wearing western-style clothes and living in a distinctly western-style house. It's true that her name is Japanese, but there is no direct evidence that that's the same name she was given at birth. She might have adopted it somewhere along the line to blend in better in Japan.

• Sunako probably born in the Meiji period. Around that time, it's not strange to have western-style house, especially wealthy family like Sunako's.