(This is based off of servamp obviously.)
Servamp, the combination of the words servant and vampire..that was exactly what (A) was now they guessed..
Anyways, let me give some context to that, (B) picked up (A) off of the streets in their daylight form (cat, doll, or something else not-human, for example) and decided to take them back home and take care of them a bit, or turn it over to some form of lost and found the next day. (A) however, kind of had other plans, because they decided to make themselves at home in their human form while (B) was out..due to them not knowing when their 'caretaker' would return..that leads to them getting caught, and now we're here..after (A) drank (B)'s blood.
Of course there are going to be others of their kind, but that's just a vague outline for the scenario.