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 wasn't just a person. Far from being just anybody, he was a "hero" who didn't understand common sense at all!! This is the story of a reincarnated person, Rain, who, fortunately or unfortunately, was raised by an unscrupulous hero and has acquired tremendous power. Soon the boy grows up and makes a harem!?

(Source: Futabasha, translated)