From the Super Mario Wiki
“Oh…♡ Such loveliness… I’ve gotten so beautiful that I’m starting to scare myself!♡”
Rosie is an extremely vain flower that lives in Flower Fields in Paper Mario. She is selfish to no end; Rosie only cares about herself, her beauty and her beautiful possessions. In fact, Rosie would rather jeopardize someone else's life than give up one of her possessions that add to her beauty in order to help save that person. This actually happens during the events of Paper Mario. During the course of the adventure, Lily, a lily pad who lives in Flower Fields, witnesses her pond dry up. Of course, without the water the pond provides, Lily would wither away and die. When Mario asks Rosie for her precious Water Stone, the only item that could save Lily's life, Rosie refuses to give up the jewel, choosing to sacrifice Lily instead of her Water Stone. Thankfully, Mario offers her a Crystal Berry from Posie; Rosie then trades her Water Stone for the Crystal Berry. However, Rosie remains vain to the end, remarking that she only saved Lily's life so there would be one more flower alive to witness her beauty.
Mario can find Rosie at the end of the western path of Flower Fields. A blue flower guards the path, demanding a Blue Berry for access to the path. Past the blue flower is a large hedge maze. If Mario can find his way through the maze, he'll find Rosie in her garden.