Sebastian I would start with his Hell backstory, because otherwise he would just be. . . someone who ate souls indiscriminately? As he implied he was as a younger demon. So I wouldn't want him any less updated than to just before taking Ciel's contract. Sera's first memory would probably be something that gave her no clues about her life past finding out Serph Sheffield was an asshole, so I could break her properly.

Three things you like and dislike to see in a canon?