The souls notice that Dante casts a shadow — no shade does — and cluster around to look, astonished. Virgil tells Dante not to slow down for their wonder: he has a mission, and they must not delay. But souls call out to him, and Dante asks Virgil's permission to listen. Virgil consents — but Dante must keep walking. This is a beautiful image of Purgatorio's rhythm: attentiveness to others, yes, but never losing momentum toward the goal.
This group is another subset of the late-repentant: those who died suddenly, violently, before they could formally confess and receive the last rites. They repented at the final breath, in extremis, and so are saved — but must wait here. Three speak. First, Jacopo del Cassero of Fano, who ran from his enemies into a marsh and bled to death, too far from anyone who could help. His death was pointless, a political assassination; his regret is that he made a wrong turn that doomed him.
The second — and the most poetically remarkable — is Buonconte da Montefeltro. His father Guido (condemned in Hell in the Inferno) sold his soul to the devil with a deathbed gift to Boniface VIII; the son, dying at the Battle of Campaldino in 1289 (the very battle where Dante himself may have fought), whispered "Mary" with his last breath and shed a single tear of contrition. This minimal act of surrender was enough. A devil comes to claim him — crying that one little tear, one murmured name, has stolen him from Hell — and Dante shows us the universe of grace: Heaven's angel carries the soul away while the furious demon can only scatter the corpse in the floodwaters of the Archiano river. One tear. One name. The arithmetic of mercy is not human arithmetic.
Third, La Pia de' Tolomei — perhaps the most quietly devastating of the three. She identifies herself in only a few lines: she was born in Siena, undone in the Maremma. She asks Dante to remember her when he rests after the long journey — adding, almost as an afterthought, that "he knows" who had wedded her with his ring, then caused her death. She does not name him, does not condemn him, does not weep. The restraint is everything. In her few words she is more fully human than pages of exposition could make her.