Sayoko Shiranagatani (白長谷小夜子, Shiranagatani Sayoko) is the first Blade Child introduced within the story in both the anime and manga. Sayoko is the reason Ayumu is introduced into the Blade Children case his brother had mentioned before leaving.
Sayoko is a calm, relaxed young woman who is less rambuctious compared to her other half-siblings. She knows little about her destiny and chooses to stay uninvolved after learning she is special. Her adoptive father made certain she remained oblivious to her true parentage as a spawn of Yaiba. Out of all the Blade Children, Sayoko has never commited any acts of violence to other humans. She is the only Blade Child not possessing cat-slit pupils and psychological issues (perhaps due to her foster father's sheltered upbringing of her).
Mr. Shiranagatani adopted Sayoko at a young age and sheltered her from the Hunters. Sayoko has no recollection of her childhood.
In the anime, she is pushed off the schoolroof she and Ayumu attend because of a Hunter posing as a professor. This is Ayumu's introduction to the Blade Children case his brother mentioned before leaving; he finds out the professor is the culprit. When found out he tries fleeing from the scene but is luckily captured. Another Hunter kills a staff member in the Sayoko residence for knowing too much. More assassination attempts are made on Sayoko, which fail due to Ayumu's intervention.