Moto Saikyou Ansatsusha no Kishi Seikatsu

He wants to find his own way of chivalry! Theor, a young boy born into a family of assassins, was branded "incompetent" by his uncle, the head of the family, and banished. Because Theor's stealth skills were so high, no one in his family realized that he was also a powerful assassin magic user...! Taking his banishment as an opportunity, Theor decides to become the knight he has always dreamed of becoming, and joins the Sixth Knights of the Kingdom of Oikot. While carrying out missions using assassin magic alongside his senior knight Lina and other unique companions, the former strongest assassin awakens to his mission to protect the people! From an assassin who takes lives to a knight who protects the people - the story of this former assassin and unrivaled knight begins!

(Source: Kadokawa, translated)