Kiriko Endo is the older sister of Kenji Endo and the mother of Kanna Endo. From a young age, she displayed a strong sense of justice and a profound interest in biology and the study of pathogens and microorganisms. Her intelligence and dedication in this field would later become central to the overarching conflict of the story.

In her youth, Kiriko was in a relationship with a man named Moroboshi. However, this relationship was destroyed when the character known as Friend orchestrated Moroboshi's death. Friend, using a fabricated prophecy that referred to Kiriko as the Holy Mother, then manipulated circumstances to marry her himself. This union resulted in the birth of her daughter, Kanna. Eventually, Kiriko became aware of Friend’s sinister creed and true nature. In 1997, she made the difficult decision to leave her daughter in the care of Kenji and their mother, Jiyo, before disappearing from their lives.

Kiriko’s role in the story is deeply tied to a deadly virus that Friend intended to use in his plan for world domination. Alongside a colleague named Yamane, she was forced against her will to help create this lethal pathogen. The virus, which caused its victims to hemorrhage blood, was responsible for a catastrophic event that resulted in the deaths of 150,000 people, a tragedy for which Kiriko carried an immense and driving sense of guilt. To counteract this, she also became the creator of the virus's cure, the vaccine, which Friend paradoxically planned to use to position himself as the hero of humanity.

Haunted by her past actions, Kiriko dedicated her life to atonement. Her development throughout the series is defined by her secret and persistent efforts to oppose Friend from the shadows. She worked to develop a vaccine that could be distributed universally. At one point, she opened a vaccine facility in America, but it was destroyed by Friend's henchmen. She was saved from this attack by the character Keroyon and took refuge in his fortress, where she continued her work to develop a new vaccine for Friend’s final virus. Displaying immense courage and a willingness to sacrifice herself for her cause, Kiriko ultimately tested the vaccine on herself to prove its effectiveness. After Friend’s plan was finally defeated, Kiriko traveled to Africa to work as a volunteer doctor, dedicating her skills to aiding the sick, and was eventually reunited there with her daughter, Kanna. Her professional abilities as a microbiologist and medical researcher are her most notable traits, shaping both the greatest threat to humanity and its ultimate salvation.