Rosaline is a character from ''Romeo and Juliet'' that we only hear about from the other characters. She doesn't actually appear in the play. Therefore, we have no data about Romeo's initial approach to her. We only know he is in love with her at the beginning of the play, and this love is obviously not mutual since Romeo is suffering.
However, once Romeo sees Juliet for the first time, he instantly forgets about Rosaline and falls madly in love with Juliet. Therefore, we can conclude that his love towards Rosaline was not very strong. On the other hand, he loves Juliet deeply, promising her to marry her and eventually, committing suicide because of her.