Individual lore: She took care of a lonely grandma all her life, she had a strong bond with her so she had a really hard time when she suddenly died. Yui spent weeks mourning her death, barely taking care of herself. Mono spotted her wandering around the grandma's house, as it got sold to new people and she was not welcomed there. After talking a bit, Mono decided to take her in. He helped her get better and then taught her some of his assassination skills.

Shared lore: Mono and Yui are a team/duo that work as guards, they're in charge of taking care of important duties from sunset to sunrise. Amor is more of a personal butler/babysitter for the daughter of the family, he spends most of his time playing and looking after her. He gets along well with everyone at the house, although he fears Yui, for she looks a bit intimidating.Forest also helps Amor take care of the beloved daughter and not only that, but she sometimes help Mono and Yui bandage their injuries from time to time.

Personality: Yui is emotionally unstable from time to time. She can get easily stressed or overwhelmed by small things if she overthinks stuff. She's awkward around creatures she doesn't know. She enjoys to be alone and is not bothered by what a others might think of her. A huge fanatic of animals and plants.