Sayaka Sonohara (園原 沙也香 Sonohara Sayaka) is the mother of Anri Sonohara and a former wielder of Saika. Despite having passed away before the start of the series, her legacy has left an impact on several key figures in Ikebukuro.
Sayaka is shown to have long black hair and brown eyes like her daughter with her eyes turning red when Saika is active. Her usual attire consists of a pink blouse and a white skirt.
Sayaka was shown to be a very loving mother while she was alive and always did her best to support her family. Even when her husband began to lash out, both verbally and physically, at her due to stress from his job, she continued to be a devoted wife to him despite the abuse.
As a wielder of Saika, Sayaka possessed many of the abilities shown by current wielders such as increased strength and skill with a sword, the ability to cause multiple blades to protrude from any part of her body, as well as being able to control anyone she had cut. Unlike her daughter, however, Sayaka never gained full control over Saika, only being able to redirect it's desire to cut away from her family.
Sayaka led a fairly uneventful life until coming into contact with Saika. It is unknown exactly how Sayaka came to possess the demon sword but considering her husband was an antique dealer, it is not difficult to think of a situation where she would come into contact with some of his more peculiar items. From here on, Sayaka would become an unwilling accomplice to the first string of Slasher attacks, injuring several people around the city.
One fateful evening, Sayaka attacked a well known Yakuza thug, Akabayashi, cutting his eye in the ensuing fight. Akabayashi's will proves to be much stronger than anticipated, however, as he resists the curse long enough for him to pull out his afflicted eye before it could spread any further. Taken aback, Saika withdraws and releases Sayaka from it's control for the evening. Sayaka is amazed that someone was able to resist Saika's influence, and asks Akabayashi to end her life while she still has control over her own body.
Unbeknownst to her, the battle had a profound impact on Akabayashi, causing him to fall in love with her. He refuses to kill her and confesses his romantic feelings instead, but is ultimately rejected when Sayaka reveals that she has a husband and daughter whom she still loves. Upon telling him her family name was Sonohara, Akabayashi laughs as Sayaka unwittingly saved her husband from being harassed by his gang. The two depart on friendly terms with Akabayashi agreeing to leave their family alone, regardless of what his boss says.
Unfortunately, Akabayashi's gang used less direct methods to try and extort the Sonoharas. They got Sayaka's husband hooked on drugs. This, coupled with the store falling into financial trouble causes him to attempt to kill Anri in order to procure some kind of insurance money. In desperation, Sayaka uses Saika to kill her husband in order to save Anri. However, as she could feel Saika taking over, Sayaka decides to take her own life before she could accidentally hurt Anri. Telling Anri she loved her one last time, Sayaka cuts her own neck and dies shortly after. Their deaths would officially be ruled as a homicide in a botched robbery.
Sayaka's death would have a much larger impact than she imagined. The incident would cause Anri to lock away her emotions almost completely. This would prove to be a blessing in disguise as this is what would allow her to maintain more control over Saika that her mother lacked. One of her victims, Haruna Niekawa, would break free of her control and go on a second Slasher spree several years later. Additionally, upon finding out his Boss's hand in Sayaka's death, Akabayashi would later indirectly betray his gang to avenge her, causing it's destruction.