We all know the tale of the Kiryu Twins, we know about that horrible snowy night which changed one of their lives forever. But little do we know that someone else in the household had their life changed as well. Kaiya Kiryu is Zero and Ichiru's younger sister. She is elastic and happy, not quite focused on her training. Ichiru is one of her best friends and she looks up to Zero, to learn from him. On one snowy night that we all know too well Kaiya comes to Zero's aid, but at a cost. That cost is what makes her human, her life changed for the worst, she is turned into the creature she was taught to hate, and fear in some cases. Waking up in an unknown room, surrounded by people she doesn't reconize, Kaiya longs for Zero, who is just down the hall. When she learns what happened to her she promises herself that she will protect the one true family she has left, Zero, from going insane. All goes well, until the enemy returns.