Who was Sunako turned by?

Question

When Sunako told her story, she mentioned that "him" was the one who turned her into a shiki.

Was it Seishirou? Is there any mention of this in the novel?

Answer 1

She was turned by an unnamed jinrou (werewolf).

According to her backstory in episode 16, when she was still a normal human, her father's friend brought a guess to visit her house. He was a foreigner/westerner and turns out to be a jinrou. In the end, she was bitten and her blood was sucked by him and she died. However, after she was put into a coffin and buried, she woke up and came back to life, as a shiki.

Researching further about her attacker, there's no more insight other than "a westerner jinrou", as shown on Wikia:

• Sunako Kirishiki

  He was a Jinrou.

• Jinrou

  There are of course jinrou overseas as well, such as the one that turned Sunako.

and other sources:

• pixiv dictionary (Japanese)

  その正体は幼少期に西洋人の人狼に襲われ、蘇生した起き上がり。

  Her real form is that she came back to life after she was attacked by a westerner jinrou when she was in her early childhood.

• Yahoo! Chiebukuro (Japanese), asking the difference between the novel, manga and anime:

  沙子は元々は人間で、（おそらく明治、大正時代と思われる）必ずしも火葬されない時代に異国から来た人狼に襲われ屍鬼になりました。

  Sunako was originally a human (probably since Meiji, Taisho period), certainly not from an era with cremation, and was turned into shiki by a jinrou coming from a foreign country.

However, it's certainly not Seishirou because he is a human, even though the attacker looks really similar to him, as noted in antiotaku's review:

The vampire who killed Sunako all those years ago bears a striking resemblance to Seishirou Kirishiki. They even dress alike. I'm guessing this isn't a coincidence.