Roses sure have come to symbolize a lot in modern western culture. But the symbolism associated with roses tends to focus on their beauty: roses mean love, or friendship, or youth. People seem content to focus on their pretty petals…and total disregard those nasty thorns.
Ha. Not Ofelia's rose:
OFELIA: A long, long time ago in a grey sad country there was a magic rose that made whoever plucked it immortal. But no one would dare go near it because its thorns were full of mortal poison. So amongst the men tales of pain and death were told in hushed voices. But there was no talk of eternal life for men fear pain more than they want immortality. So every day the rose wilted, unable to bequeath its gift to anyone. Alone and forgotten at the top of that mountain…forgotten until the end of time.
This rose might offer eternal life, but it's thorny and poisonous mountain promises enough pain that men (who seem to always be seeking immortality: from the Fountain of Youth to cryogenic freezing to Botox) won't even go near it.
So what does this mean?
The rose, which is often a feminine symbol, could represent Ofelia's potential journey to adulthood. But Ofelia's journey into adolescence is fraught with pain and fear—she doesn't want to attain a womanhood like shared by Mercedes and Carmen. The rose's thorns are doing symbolic double-duty: they both represent the scary physical changes like puberty (specifically menstruation), and men, who pollute the world of women with war and misogyny
But Ofelia never makes this journey. She never gets the chance. And while she's alive she spends her time running in the opposite direction of adulthood—back into the innocence with which she was born, an innocence alluded to by the narrator's description of the pain- and lie-free Underground Realm.