Muteki no Ansatsu Madoushi, Last Boss Musume to Tabi wo Suru @comic

Brad Lester, once renowned as the strongest mage, vanished from public eye after the tragic death of his young daughter. While he is living a quiet life as a lowly day laborer, his beloved wife is murdered by the "World Peace Promotion Association," an organization seeking his power. Filled with despair and rage after losing everything, Brad sets out for revenge—only to find Valburga, the leader of the Association, has been transformed into a little girl!? Furthermore, she has lost her memories and has nowhere to go. Haunted by thoughts of his late daughter, Brad resolves to help his arch-nemesis regain her memories. Visiting the Olford Ancient Magic Research Institute, the mouse-worshiping town of Brooke, and the blizzard-shrouded village of Lutz, their long journey across the land begins. An adventure fantasy of the strongest mage and his "child" companion!!

(Source: TO Books, translated)