Genova no Deshi: 10-byou shika Tatakaenai Majutsushi, Nochi no "Maou" wo Sodateru

"Genova no Deshi" is a fantasy adventure story about Valen, a former powerful magician, and Leah, a cross-dressing girl with a secret. After losing his beloved master, his eyesight, and his magical powers in an incident, Valen can now only use his former power for 10 seconds at a time, and makes a living as a flute player. Leah, like an uninvited wife, suddenly becomes his apprentice. When Leah informs Valen that his master, whom he thought dead, is still alive, he sets out on a journey with her.

(Source: Natalie, translated)