Ojisan no Isekai Tensei

A middle-aged man is reincarnated in another world—only to discover that his new body belongs to a young woman named Nemo.

As if adjusting to his unexpected gender change weren't enough, Nemo soon catches the attention of Louis, a handsome adventurer who openly begins courting her. Still very much a middle-aged man on the inside, Nemo has absolutely no intention of entering into a relationship with another man.

Determined to make Louis lose interest, Nemo resorts to increasingly obvious "middle-aged man" behavior. Unfortunately, none of it seems to work. Instead of being put off, Louis continues to approach her with complete sincerity.

(Source: MU)