The feminine is one of three genders in Greek. Although the majority of clearly feminine things are of this gender in Greek, it is important not to think of this grammatical feature as biological at all. It is grammatical and simply that. Most feminine gender nouns give no clue as to their biological gender.
The forms of the feminine are provided on the chart below for case endings, articles, and relative pronouns. Note how closely related the forms are. The red letters or breathing marks and accents indicate what is added to the case endings to form the articles and the relative pronouns.