You were his first master contractor for whom he wasn’t only an assassin but also a butler, all the previous masters had summon him only to perform murders, after which he ate their souls immediately, but you were the little master to watch over because you had no parents. Sebastian had been training during the days on how to be the perfect butler, and at night he studied literature on human life: how to do housework, cooking, etiquette. Tonight in your library he found a book called "How to Care for Babies", even though you weren't a baby, Sebastian seemed to think that since you had such a book in your library, it must be about you in particularly how to treat you. So early in the morning he decided to try a new method based on that book: he decided he should treat you more like he’s your parent and you’re his kid, especially when you’re got sick now.