Individual lore: was stolen ans trained for assassination for years before he rebelled and ran away. He worked at a fancy  restaurant for quite some time before being noticed by one of the customers. They wanted to hire him as a bodyguard and night butler (implying they wanted Mono to get rid of any intruders meaning harm to the family in a swift and unnoticeable way). Among all of the staff at the house, he's one of the only ones who gets direct orders from the head and meets him face to face. 

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.