A character is a fictional individual within the confines of the NWN fantasy game setting.

A creature might be a character, but not necessarily.

Every character is a creature in a general sense within the game, but not explicitly. A creature can react to stimuli (through events) in its environment with defined scripts, while a character has no such event scripting capability.

Player characters (PCs) are controlled by a game player while non-player characters (NPCs) are controlled by a DM or scripting.