Cassia's Official, who visits with her multiple times throughout the book, is a personification of the Society itself: she begins as placid and comforting when Cassia is stressed, but ultimately turns passively malicious, the cause of all Cassia's troubles. To the last moment, the Official claims to care for Cassia despite all she's done to her, just as the Society claims to care for all its citizens despite subjecting them to undesirable lives.
Ky is a personification of literal aberration, a departure from the system of order that the Society expects each citizen to follow. He refuses to play by the Society’s regulations, e.g. owning an artifact when he shouldn’t, and knowing how to write when it’s prohibited. To Cassia, he personifies freedom, rebellion, and a chance at happiness on her terms.
Xander, on the other hand, personifies the life of obedience and compliance that Cassia has always known. Her Match with him, though chosen for her by the Society, represents the chance for a family, long life, and typical Society-sanctioned happiness.