A Satarran agent who wiped the Enterprise crew's memories and posed as their first officer to turn the ship into a weapon.
Kieran MacDuff is the identity assumed by an alien of the Satarran species. After a scan from an unknown vessel erases the crew's memories of who they are, MacDuff appears among them as the ship's first officer, a role nobody can recall him filling.
In Conundrum, MacDuff uses the crew's confusion and falsified computer records to convince them that they are at war with the Lysians, pushing the Enterprise to destroy the Lysian central command. Picard balks at attacking a nearly defenseless enemy, the deception unravels, and MacDuff is subdued when he tries to finish the attack himself.