Aidou Kou, also known as Ai-Tong in the English translation, is the youngest of the five daughters of the Kou family. She appears in volume 14 of the original series and Tasuki's one-shot, Orega Onnagirai ni Natta Wake, but is not present in the anime adaptation.
Upon her entrance, Aidou’s character is shown as hardworking, warm, stern where she needs to be and sarcastic when she wants to. Although she only appears briefly, her presence helps clear up the cause of why Tasuki “hates girls.”
Growing up with a passive father figure left more than a few marks on the Kou family, leaving Aidou and her siblings to work hard for their livelihoods and their mother to take on a more dominant role in the household. Being surrounded by girls with no proper figure to lead Tasuki through his teenage years forced him to find guidance elsewhere. But not for Aidou, who seems to be empowered by the strong women in her life.
While it remains unclear where the rest of the sisters ended up after leaving home, Aidou proves her devotion to her family by staying behind and helping her parents in their later years.
But her appreciation does not stop with her parents. When she meets Miaka and Tamahome, she would not stop thanking them for providing support and purpose to her little brother’s life.
Like most siblings, she knows her brother well and can tell when he’s feeling off, which she’s not afraid of pointing out to the headstrong warrior.
Aidou appears in Tasuki's one-shot.
Fushigi Yuugi: The Mysterious Play
Miaka and Tamahome, or Taka at the time, first encountered Aidou upon their first return to the Universe of the Four Gods, two years after the original story wrapped.
The two were attacked by one of Tenkou’s demons when a mature Tasuki stepped in to save the defenseless couple. Their reunion was interrupted by Aidou, who hurled a faggot of firewood at her younger brother. After spouting insults at Tasuki for slacking off, she noticed his friends and immediately invited them into the family home for supper.
When they were settled and the meal was cooked, another one of Tenkou’s demons burst from the table and made for Miaka and Tamahome. Acting fast, Tasuki used his Tessen to destroy the demon, evoking his mother’s wrath for using a weapon at the dinner table. Aidou, unaffected, chastised their father for not reacting appropriately—or at all.
Shortly after the incident, Miaka and Tamahome were abruptly sent back to the real world, leaving the Kou family behind. Aidou did not appear again until Perfect World magazine Volume 5 in a small vignette detailing the lives of Tasuki and his sisters.