Futago no Omamori

Mamori is a solitary girl living a lonely existence. Her parents vanished into thin air, and her older brother—her only remaining relative—has gone missing.

However, acting on a recommendation found in one of the periodic letters she still receives from her brother, she applies for a job as a bodyguard for a pair of wealthy twins.

She heads to the interview and, despite being rejected by the twins themselves, manages to pass. But there is a catch: her new workplace turns out to be the all-boys school that the twins attend...