Kayo's family a few generations before she was born went under immense pressure of the other shinobi clans as her family were to be discarded. However unleased a cursed heirloom and thus many shinobi died under the full moon. Kayo's family began to seek refuge, not wanting to be killed. Zixin's grandfather accepted their family as servants and thus many shinobis could only hold their bitter rage in their hearts against her family, unable to do anything as Zixin's family was powerful. They stayed hidden and hopefully forgotten by the ninja clans.
Kayo is a ninja serving Miyu and Zixin's family, serving them for a few generations. Kayo has taught Miyu and is considered her teacher. Unlike other ninjas, Kayo prefers housekeeping than actual ninja related work as fighting is not her forte. However she is the owner of the full moon dagger, and her summon has an overwhelming ability to kill.
Kayo marries Yuji, an elite ninja from a different ninja clan and through him finds herself wind up in a world of conspiracies and lies used to destroy the people she loves. She is unaware of her past life.
Yuji is a son of the White Fang Ninja clan. Grew up in a rather large household with several siblings serving a noble house. He grew in a very disciplined home, learning ninjutsu and learning to be immune to many poisons through it.
He became one of the elite ninjas. His family hopes to make their ninja family more known by having their middle son marry Kayo. Yuji accepts as he’d rather serve a powerful and strong man like Zixin then marrying another who was not worth serving. However he looks down on his new wife as he believes her abilities are below par compared to his, and thus takes it upon himself to force her to learn to better her combat skills.
However he is insecure, and believes that if he is strong there is nothing in stopping him.... except for his wife as he is confused about what he feels for her and about his loyalties.
Yuji become his family's true intentions of having him marry his wife, hoping to obtain her powerful oni under her possession.