Its about Tzuyu being a soldier, she was experimented by his father, and commander at the same time, when she's a kid. And as a child, she was sent to kill Sana's father who's at that time is a military commander, but couldn't accomplish her mission. A friend of Sana's father kept Tzuyu and took care of her.
Tzuyu and Sana met at a school where Tzuyu's "father" enrolled her to make Tzuyu have a normal life.
I'm sorry, I just couldnt remember the title.
Thanks for understanding.