Taeko Hirukawa is introduced in Episode 6. She deserved better. Her current age is 17 years. The timing of events leads viewers to assume she is the granddaughter that the old woman is speaking about missing, but that turns out to be Midori.
As a recovered Ichi and Ushi run past her on the street she has a flashback to when she was younger. Then, Taeko's father rescued her from some bullies. Taeko's mother also appears in a flashback when she was graduating from elementary school.
She remembers her parents taking her to an arcade. She also remembers them arguing over whether to buy a new house so that Taeko could have a larger room for studying in school.
She claimed that she was happy being in the house she was born in. That opinion seemed to change as she grew older though. On her father's birthday, after they moved into their new house, she decided to set up a birthday surprise on his computer, but found a strange file instead. After looking through it, Taeko discovers that her father has installed a camera into her new room and had been secretly taking pictures of her undressing, which explains his fetish of being called "Daddy."
After which, it cuts to her considering suicide via drowning up until she notices the old woman being taken away from the river. She then pleads for Lil' Slugger to make her "forget." Once she gets hit in the head, she proceeds to forget everything: Her relationship with her father, what happened before, and who she is.
Her and Tsukiko's second attack are what tells Mitsuhiro that more than one attack can occur at the same time. Any episodes after 6 show her sitting at a swing set with a smile on her face, most likely in ignorance of her father's actions. It is also worth noting her appearance in the intro depicts her drowning underwater.