Satoko Kirigaya is the female protagonist of the manga Firefly Wedding. She is the eldest daughter of the respected Kirigaya family during the Meiji era. While she is known for her exceptional beauty, intelligence, and grace, she was born with a serious heart condition that has left her physically weak and with a limited life expectancy, a fact that doctors have told her means she may not live to become an adult. Because of her frailty, she has spent most of her life confined to her home, leading to a somewhat sheltered existence.

Despite her noble status and striking appearance, which has earned her a reputation as the most beautiful girl in her town, Satoko's primary motivation in life is not personal happiness but duty. Her sole dream is to enter into a marriage that will benefit her family, as a way to repay her father for his kindness and to fulfill his wish for her to be cared for before she dies. However, she is principled and refuses to deceive potential suitors, always informing them of her illness, which often causes marriage proposals to fall through.

In terms of personality, Satoko presents a composed and polite demeanor, marked by a fierce determination and an acute awareness of her appearance and posture. She is deeply considerate of others, showing concern for her guardians and taking the time to cultivate flowers as gifts. Yet beneath this confident exterior, she harbors a profound self-loathing due to her condition. Feeling like a burden to her family, she has gone so far as to compose a note expressing a wish for her body to be used for scientific research after her death, hoping to minimize funeral expenses. Her life is defined by the contradiction of striving to be perfect for others while despising herself for her physical weakness.

Satoko’s role in the story is set in motion when she is kidnapped by a group of men while visiting town with her guardian, Koutaro. Her captors have hired an assassin, Shinpei Goto, to end her life. Facing death, Satoko uses her wits to survive, making a desperate gamble: she pretends to fall in love with him and proposes marriage on the spot. He accepts, and this unlikely contract marriage becomes the central relationship of the series, forcing her into a precarious situation where she must maintain the ruse while escaping with a man who is both her protector and a genuine threat.

Her key relationships are complex and central to her development. Her bond with Shinpei Goto begins with fear and manipulation, as she is initially terrified of his violent and unstable nature. However, as he takes her away from her sheltered life and shows her new experiences, her feelings evolve into genuine love, even as she discovers his obsessive and yandere tendencies. Her relationship with her family is also fraught with difficulty. While she is devoted to her father, Jubei, it is later revealed that he played a role in the events that led to her being sent away. Her stepmother harbors deep jealousy and resentment towards her, subjecting her to verbal abuse and treating her as a tool for family profit, while her half-sister, Rie, is favored. She is also guarded by Koutaro Ogawa, her childhood friend, who accompanies her into town.

Throughout the narrative, Satoko undergoes significant development. Initially a passive, obedient girl who had resigned herself to dying for her family's sake, she demonstrates a hidden well of courage and quick thinking when faced with the immediate threat of death. Her journey with Shinpei exposes her to the outside world and the harsh realities of life, which gradually frees her from the psychological control of her stepmother and allows her to find a will to live for herself. She grows from a terminally ill noblewoman who saw herself as property into a young woman capable of fighting for her own desires and love.

Despite her physical frailty, Satoko’s notable abilities are primarily mental and emotional. She possesses exceptional intelligence and an unwavering determination to achieve her goals, which allows her to quickly assess a life-or-death situation and formulate a plan to survive. She is a convincing liar and actor, able to maintain the pretense of being in love with a dangerous assassin long enough to secure her escape. She is also acutely observant and persuasive, able to see through Shinpei's erratic moods and appeal to his deeper nature. While she cannot fight, her strength lies in her resolve, as she constantly chooses life and overcomes her fear moment by moment. Physically, she is marked by a large scar on her chest from a childhood heart surgery, a permanent reminder of her condition.