Ochikobore Eji Minarai no Shounen. (Jitsu wa) Saikyou Saiaku no Ansatsusha. The Lazy Boy Is, in Fact, the Strongest and Most Brutal Assassin.

Synopsis

In a certain kingdom's capital, war is approaching. A young boy named Il, an apprentice guard, is treated as a useless person who can't do his job properly, living a lazy life while constantly relying on his stepsister. Amidst this, a certain rumor is spreading throughout the city. It is said that a mysterious group of assassins known as the Night Dwellers are operating in the shadows. This is the story of the worst assassin among them. (Source: Ichijinsha, translated)