Who is Rosaline?

At the start of Romeo and Juliet, Romeo is lovesick over a woman who doesn't share his feelings. Though he doesn't name her initially, later, Benvolio reveals her identity as Rosaline, a Capulet. Romeo learns that Rosaline will be at the Capulet ball, and Benvolio suggests he attend to find someone new. Although Rosaline never appears on stage, her rejection prompts Romeo to attend the ball, setting the stage for his life-changing meeting with Juliet.