Yuyami is a rouge diviner who runs a brothel where girls are kidnapped and then broken down into sexual submission, thanks to the help of several incubi which she has brefriended.
Yuyami kills anyone who gets in her way, including the best friend of the game's protagonist - Towako Yagyu. Years before this, when she was a terrorist, she also murdered both of Towako's parents while hijacking a plane; an act which she deeply gloats about.
At the start of the game, Towako and her summoned deity, Vanguard, battle Yuyami and her summoned deity, Jormungand. Soon, however, Towako falls under the influence of the incubi and is put through numerous humiliating acts as Yuyami's sexual slave.
Through the help of her friends, Towako is finally able to break free from this hell and battles her arch enemy one last time. Using all of her power, Towako deals a blow to Yumami, causing the villainess to burst into a thousand pieces.
At the end of the game, it is revealed that Jormungand was not serving Yuyami, but in fact, the other way around; adding that the evil woman forged a pact with the demons in exchange for immortality.