The Sixth Faction by Veronica Roth

In this alternate-universe reimagining of Divergent, a shocking tragedy at the Choosing Ceremony disrupts Beatrice "Tris" Prior's plans and forces her into an unexpected faction choice. This dangerous new path plunges her into an underground rebellion alongside a mysterious boy known only by a number. As secrets unravel, Tris must navigate a world where memory is weaponized for total societal control.