Hisako Yao is the main antagonist in Forbidden Siren. Once a villager of Hanuda in 684 AD, she was cursed by Datatsushi into immortality.
Hisako was a nun of the local Mana religion. She wore a crimson robe with salmon pink accents. She also wore the traditional Mana Cross around her neck. Initially, her hair was kept tied into a bun, but when she became aware of her true purpose, she let her hair flow freely.
Hisako was originally a villager of Hanuda in 684 AD. She was the one who discovered Datatsushi's corpse during the great famine of the time. Along with two unnamed villagers, she initiates the devouring of the alien's corpse. From this, Datatsushi emanates an intense scream (what would be known as the Siren). Hisako appeared visibly shaken by the experience while the others merely covered their ears. Subsequently, she was cursed with immortality unless Datatsushi is summoned back to life.
In the beginning of the game, Hisako partakes a sacrificial ritual that was meant to summon Datatsushi. Kyoya's sudden appearance interrupts the ritual and the Siren causes the dead villagers to rise as Shibito. She encounters Kyoya and helps him master the phenomenon of Sightjacking. Together, they head to Irazu Valley Church near Karuwari. In Irazu Valley Church, Hisako discovers a hymn book that contains the message "Awaken", written in Mana.
From this point onward, Hisako gains insight into her true purpose. Her previous self gives the head of Datatsushi while using Miyako as a sacrifice. With the help of Jun Kajiro, Hisako kidnaps Miyako and successfully makes her a sacrificial lamb, summoning a premature form of Datatsushi.
In the game's finale, she offers herself and eventually summons Datatsushi in its full form. However, it was decapitated by Kyoya, causing her immortality and youthful appearance to be reversed. She picks up Datatsushi's head and ages rapidly before falling into a dimensional vortex, travelling through time. She gives her younger self the alien god's head, resulting in Datasushi's return.