Level 0 no Maou-sama, Isekai de Boukensha wo Hajimemasu

Ishuto was the strongest Demon Lord in the Dark Continent until he was defeated by the heroes. But instead of dying, he reincarnated into another world as a young man, while still keeping his powers. "I will start from scratch in this world and this time it'll be mine!". The elite adventurer Airis was charmed by Ishuto's strength and became his guardian so he could enter the adventurer training school, but due to his otherworldly powers, he graduated the very first day, receiving the unprecedented title of "level 0". He's a maverick with Demon Lord powers that has no match in this world...

(Source: MANGA UP!)