In life, she was Usuyuki, a woman who obsessed over Shinza, a man who never loved her. However, when scorned that he loved another woman instead of her, Usuyuki incinerated him and those at an engagement party in a suicidal crime of passion. But once witnessing that Shinza would still love another, Usuyuki's emotions corrupted her soul and turned her into Dayu, bonding Shinza's soul into a shamisen that she carries with her, creating eerie and unsettling songs that become a means to soothe Dokoku's rage. However, she has trouble getting along with the Ayakashi because of her life as a human, which she makes attempts to reclaim.
One failed attempt results with her being saved by Juzo, developing feelings for the Gedou as she begins to defy Dokoku's orders as well. Though she claimed her feelings for Shinza had long died, after becoming a rogue, Dayu refuses to give him up as it would end her existence and she doesn't want Shinza's soul to be reunited with his love. Dokoku angry that she did not killed Juzo as he ordered her to, he burned her shamisen, making Shinza moan. She told Juzo her feelings for Shinza had disappeared but Juzo did not believe this true. Soon after Akumaro's demise, Dayu returns to the Rokumon Junk to assist in Dokoku's reawakening.
Dayu deliberately has Shinken Pink taking care of her shamisen. which discards her of her past life, but it also revives Dokoku. She later uses the last of her power to restore Dokoku's body with only her kimono remaining, which Dokoku later throws into the Sanzu River. In battle, Dayu wielded a shortsword hidden in her shamisen, with the headstock as its hilt.
Super Hero Taisen Z
Dayu was part of an Army of Resurrected (mostly Super Sentai) monsters led by Space Raider after the revival of Madou Demon King Psycho. They proceeded to attack the gathering of Kamen Riders and Super Sentai, giving them a hard time. The heroes of recent years were in a pinch until the arrival of a force of Sentai and Rider reinforcements led by Akarenger and Kamen Rider Ichigo. The combined forces of superheroes were able to turn the tide against the Madou army. They lined up and used their various finisher attacks to destroy the revived monsters.