What does it mean to be someone's Valentine?

When Valentine was in prison, a very young girl from a family he had befriended continued to visit him and send him letters. Just before he was martyred he reportedly sent her a final letter asking her to remain faithful to God. He signed it "From your Valentine." From that the custom of sending "valentines" grew

Valentine became the patron saint of lovers because, after the Roman Emperor outlawed marriage, Valentine continued to marry couples in secret. The emperor was having problems raising an army because he could not draft married men.