Partner characters, which may simply be referred to as partners or non-player characters (NPCs) are characters from throughout the Resident Evil series which serve to assist or simply just follow the main playable character. Up until and beyond the implementation of co-op in Resident Evil: The Umbrella Chronicles there are at least a few AI partners in each installment of the series. Although in many games these characters are later briefly playable or are playable through the player's choice. NPCs that the player never controls in any game of the series do appear here and there but are usually only able to speak to the player character and don't assist in any way.