CRIMSON EMPIRE Manga
Sheila is a housemaid and exclusively serves Edvard, a prince. She also works as his security guard because of her past assassin training. She is all-purpose at work, but awkward when she is off. Although Sheila is competent at battles and operations, but she isn't good at love and establishing friendships. She, however, needs to use the various people around her to have Edvard take over the throne...
From Deka Scanlation:
This series is about a girl who when she was a kid, she was sold by her family as a slave. She met a demon while she was at the slaver's place who told her that she would be successful. She was later bought by an assassins guild and was trained to be an assassin. But after a little fight with the guild master, a legendary assassin, she and her 2 other friends were sold again. Now, to be bodyguards for the nobilities.