Megumin

Megumin is a 13-year-old (14 and 15 later on), archwizard who is part of the crimson demons race; powerful humanoids who possess black hair, red eyes, and chuunibyou characteristics. Megumin only knows a single spell, an incredibly powerful Explosion spell that immediately incapacitates her for the rest of the day when cast, and refuses to learn any other skills.

From Wikipedia