Piccarda, sister of Corso Donati and of Dante's friend Forese Donati, is the first character Dante encounters in Paradise. She is on the Sphere of the Moon, the lowest sphere of Heaven. Piccarda explains to Dante that her placement is due to "vows neglected and, in part, no longer valid." When she was alive, Piccarda, a nun, was forcibly removed from her convent by her brother Corso, in order to marry her to a Florentine man and further her family's political interests. She died soon after her wedding. In her acquiescence to her brother's wishes, though forced, she neglected her vows to God.
Through Dante's encounter with Piccarda, we first begin to learn about the nature of Heaven. For example, we learn that souls in Heaven become much more beautiful than they were on Earth; in fact, it takes Dante a while to actually recognize Piccarda as the woman he knew. In higher spheres, souls become so beautiful they cease to resemble their earthly selves. Piccarda is the only person Dante will recognize, unaided, in Heaven.
Dante asks Piccarda if she does not long to be placed higher in Heaven. Her answer (she does not wish to be higher) highlights another important point. According to Piccarda, blessed souls long only for what they have, and so their wills are entirely in agreement with that of God. If they desired to be higher in heaven, then their wish would differ from God's will, which is an impossibility. Though they know there are others in higher spheres of Heaven, they rejoice in their placement.