Ojisan no Isekai Tensei

A middle-aged man is reincarnated in another world in the body of a young woman named Nemo. While adjusting to her unexpected change in gender, Nemo draws the attention of Louis, a handsome adventurer who begins courting her openly. Still a middle-aged man on the inside, Nemo has no interest in a relationship with another man. She tries to drive Louis away by behaving in increasingly obvious ways that reflect her middle-aged self, but he remains undeterred and continues to approach her sincerely.