The members of Class Zero are the children of a mad sorceress-doctor who is, as you can guess, not all that she seems. The kids get along decently with one another, but not at all with outsiders; Class Zeros members were accepted to the academy without entrance exams at the insistence of their mother, so talking with NPCs as any member of Class Zero often yields deriding comments and snark about how the children dont fit in with the rest of Akademia, a university breeding children for war.
At any time while wandering the world, you control one member of Class Zero as he or she talks to NPCs, either to receive a fetch quest, or simply to chat. Conversations may have different outcomes depending on which character you are currently controlling. For example, if you talk to Dr. Arecia Al-Rashia as Rem or Machina--the two new members of Class Zero who dont fall under her affection--she greets you with disdain. Approach her as Ace or Cater, however, and she warmly engages with you. Elsewhere, some side quests require that you talk to certain people or complete tasks as Rem, who is a favorite among her peers. These subtle nuances make all 14 characters matter. Each has his or her own distinct personality, and while some Class Zero members are less bearable than others, they have great conversations when interacting as a group. Watching the airheaded Cinque try and reason with the know-it-all Trey, all while snarky Sice tells them both to shut up, is always entertaining, as is seeing opposing personalities like Cater and Queen--one sassy, and one cold and rational--pair up to solve problems.