Sekai Saikou no Ansatsusha, Isekai Kizoku ni Tensei suru

Synopsis

Using his mana vision, Lugh attempts to find someone who can be both his personal servant and his assistant during assassinations. Sure enough, Lugh finds such a person in the form of Tarte, a girl from a poor background who had made her way to the Tuatha Dé region in hopes for a better life. After rescuing her from a pack of wolves, Lugh brings Tarte to his home and gets permission from his father to train her as an assassin. When he reaches the age of 12, Lugh finds that Tarte has become skilled in spear fighting, but has trouble using smaller weapons like knives. To solve this problem, Lugh creates a special, retractable spear that Tarte can hide beneath her clothes. With a good weapon at her disposal, Lugh believes Tarte has become a valuable asset in his mission to kill the Hero.

(Source: Wikipedia)