In Shakespeare's play, Romeo and Juliet, the audience never meets Rosaline. We know early on that Romeo is in love with her to no avail. His heart (he thinks) is breaking for this woman who does not love him.
Romeo's friend are tired of seeing Romeo so down, so they encourage him to get over Rosaline. Benvolio suggests (foreshadowing) that Romeo may meet a beautiful woman at the Capulet's party who will outshine Rosaline in his heart, and this is, in fact, what happens. When Romeo and his friends crash the Capulet party, Romeo decides to attend only because he knows the Rosaline will be in attendance. (Rosaline is Capulet's niece.) Her real importance is that when Romeo goes to the party to catch a glimpse of her, instead Romeo sees Juliet, and he is forever changed.
We find out that Rosaline does not wish to marry, but has chosen instead to join a convent to lead a chaste and pious life. What makes Rosaline a heroine is that she has chosen to give herself to God's work rather to flirt, dance and be wooed as most of the other young women around her. In a male-dominated society, and in the face of the young and handsome Romeo's pursuit, her commitment to join a convent remains firm. She cannot be swayed, but is dedicated to serve. This is honorable, and I would assume that her devotion is what makes her a heroine.