He Was Born Without Magic But Became The Most Powerful Magician

The world as we know it is actually just half the story, as Takeshi Nanase finds out abruptly one summer morning. On his way to kendo practice, Takeshi comes across an unconscious girl in a uniform he doesn't recognize. Takeshi does the decent thing and saves her, and in return the girl wakes up and accidentally turns him into a magic-user.

Anime: Mahou Sensou