It's a little vague as to what you mean by "mess up the story," but I can certainly explain Rosaline's role in the story. Rosaline, just like Juliet, is a relation of the Capulets, although probably a much more distant relation unlike Juliet, just like Mercutio is also a distant Capulet relation. We also learn in the first few scenes that she is someone Romeo has fallen in love with but who does not return his affection. She has a very minor role and is never mentioned again after the first act nor does the audience ever actually meet her. However, despite her minor role, she actually serves a very useful purpose in helping to characterize Romeo. Without Rosaline as a minor point in the plot, we would not have seen as easily seen just exactly how young in terms of maturity Romeo really is.
Romeo's fickle nature, meaning his sudden switch from pining over Rosaline to worshiping Juliet, testifies to Romeo's lack of mental and emotional maturity. More specifically, his fickle nature shows that he is far too young to truly know what real love is. Romeo's conversation with Friar Laurence about how Romeo has forgotten Rosaline in favor of Juliet confirms Romeo's immaturity. As Friar Laurence phrases it, "Young men's love then lies / Not truly in their hearts, but in their eyes" (II.iii.69-70). Seeing Romeo's immaturity helps prepare us for all of the emotional decisions he makes later on that lead to his death, especially his decision to avenge himself on Tybalt for Mercutio's death.
'Romeo and Juliet' is one of Shakepeare's most widely known piece of work. As a romantic tragedy, 'Romeo and Juliet' has touched the hearts of many.
Rosaline was introduced in 'Romeo and Juliet' as Romeo's love interest. Romeo was seen as completely smithin and infatauted with Rosaline. Yet just a few scenes later, we see Romeo forgetting her completely:
Romeo has, at this moment, fallen in love with Juliet.
Rosaline acts as a voice in the background constantly creating doubt for in us about Romeo's love for Juliet. Since he was able to so quickly forget about his 'love' for Rosaline, one can not help but wonder if he is or ever was truly in love with Juliet.