Kikaku Hazure no Eiyuu ni Sodaterareta, Joushiki Hazure no Mahou Kenshi

A young man who died in an accident while protecting a girl reincarnated in another world and was picked up by an old man. However, the old man was not just a regular person. Far from it, he was actually a "hero" who couldn't understand common sense at all! Soon the boy grows up and creates a harem?!

(Source: Futabasha, translated)