Himekishi-sama no Himo

Matthew is just another parasite in the chaotic dungeon city of Gray Neighbor. People say he's a lazy weakling who gambles and drinks away all the money he gets from his keeper, princess knight Arwin. But the truth is, no one in this town knows what he's really like—a ruthless protector who works in the shadows and will do whatever it takes for the sake of his mistress.
(Source: Yen Press)