She is the niece of Lord Capulet

. Although an unseen character, her role is important: Romeo's unrequited love for Rosaline leads him to try to catch a glimpse of her at a gathering hosted by the Capulet family, during which he first spots Juliet.

Is Romeo actually in love with Rosaline?

Romeo is not in love with Rosaline, but he thinks he loves her because he is in love with the idea of love. He pines for a love object and conveniently finds it in Rosaline, a young woman he worships from afar who doesn't return the favor. Romeo is an adolescent who turns his infatuation for Rosaline into high drama.

Why is Rosaline not interested in Romeo?

Rosaline has vowed never to give up her "chastity" and to remain celibate. Of course, Romeo finds this a terrible waste and he complains about it for several lines. It tells us that Romeo is quite a sensitive young man and that it is perfectly plausible that, after seeing Juliet, he falls in love at first sight.