Kakushite Mahoutsukai Noi Garene wa 100-nengo, Hanayome to Natta

Noi Galeneé, the country's greatest magician, lives alone in the mountains on the outskirts of town. One day, she receives an order from her country to watch over and defeat a boy who is possessed by the Demon King. However, Noi gives the boy the name "Cardia" and accepts him as her apprentice. The two spend warm, peaceful days together. However, the day of the Demon King's awakening draws closer with each passing moment...

The century-long story of the solitary magician Noi and the lonely boy Cardia begins now.

(Source: Kadokawa, translated)