Shinsouban Crimson Empire: Circumstances to Serve a Noble

The heroine is head maid to Prince Edvard, and her loyalty to him is absolute. The throne that should be his by right is being contested by Prince Justin—whose mother held lower rank, giving Edvard the stronger claim—and Justin's solution is to send assassins. Every assassin sent has been defeated by the head maid before reaching Edvard. This pattern continues. How long it can continue is the question the story is asking.