In general, the non-player characters can be thought of in three ways: helping (protagonists), hindering (antagonists), or neutral. (Most neutral NPCs will be the one-dialog-line commoners, so you won't see any prominent neutral NPCs in this section.)

Among those who help the player characters along the way:
Kory, Ko's older brother
Wallace, a Drythian activist in league with Enna
Aequus, Dell's dragon steward

Among those who work against the player characters at some point:
Kydran, the main antagonist, former Antarean general
Dantalian, the president of Pylos
Hyde, Dantalian's right-hand man
Miriel, Pylos' lead eumoneticist
Benivalle, the usurper of Cade's family's status
Tyrus, Protean gate-keeper and Brooke's mentor
Paerda, a Gygan drug lord