- Saiko Akamine.(src)
Takahisa Tajima (adopted son)
Saiko Akamine (赤嶺 彩子 Akamine Saiko) is the nurse at the Kouryoukan Academy and the current guardian of Takahisa Tajima. In Sin, Damnation and the Atonement Girl arc, she acts as a sub character, but becomes the main drive during her final appearance in the plot.
At school, Saiko wears a black camisole and stockings, and a red skirt and high-heels, and wears a white lab coat. Days off, she wears the same outfit, except she wears a jacket or without it.
Saiko is described as a lewd school nurse who spies around the campus for virgins. When alone, she even uses sex to satisfy both herself and Takahisa Tajima. This trait of her is probably due to her time as a street biker from a few years before the events of Sin, Damnation and the Atonement Girl arc. After taking Takahisa in however, she abandoned her old lifestyle and started working to raise him. The fact that she became a nurse in Kouryoukan Academy is probably to keep observation on Takahisa at all time, even though she isn't good for it.
She also realized that Takahisa had taken a liking to Yukiko and created many opportunities to make them come together. Despite having a somewhat bitter relationship with Yukiko's aunt, Mutsuko in the past, she still welcomes Yukiko like family.
Saiko is Kouryoukan's nurse who frequently runs around the campus (with full of killing intent) searching for a delinquent whose name is Takahisa. It is later revealed that Saiko is an ex-student of Kouryoukan Academy. In the past, she shared a rivalry relationship with Mutsuko Hirohara, a top student at school who always prevailed against her. After graduation (if any), she became the gang leader of an all girl biker gang called Kurenai Tenyo (紅天女 Kurenai Tenyo, lit. Red Fairies), which was dubbed as Crimson in katakana. During one of her escape from the police, she found Takahisa on the street, striving for survival. Sympathized, she took him in, working as a nurse in her old school to raise him.
Sin, Damnation and the Atonment Girl
Saiko appears many times in the story, playing the role of tension relieving. However, in her final appearance on October 28, she became a major plot driver. While taking about holding an event with Yukiko and Takahisa, Misao Kusakabe broke the border between Red Night and the real world to manifest herself into the present. As soon as she appeared, she attacked Takahisa without saying anything. Saiko stepped between them and took the hit, resulting in both her neck and heart being stabbed. She died in Takahisa arms shortly afterwards. This turn of events enraged Takahisa and caused him to go on a rampage, destroying half of the city. Later on, when the events of Red Night ended, she was resurrected as Kukuri Tachibana recreated the world. Apparently, her memories of being killed by Misao was erased.
In the anime, her role is nearly the same, but her lewd personality is cut out from the adaption. Her death is also more violent. While Takahisa planned all of the three eating yakiniziku together, he and Yukiko found her stabbed and hanged, with Superbia near her, which enraged Takahisa. In the end, she is seen alive with Takahisa and Yukiko in another world.
Takahisa Tajima: She adopted Takahisa when she found him on the street, and she showed be very caring about him, as she sacrificed herself to protect him from Misao. She also tried to make him and Yukiko together, despite the rivalry she had with her aunt in the past. In the anime, Takahisa is more distant with her, but still cared for her.
Yukiko Hirohara: She had a rivalry with her aunt in the past, but Saiko tried a lot to make her and Takahisa together and acts friendly with her.