Aiden was a witcher of the Cat School and a good friend of Lambert. The two met when they had both been hired by different parties to deal with the Ogre of Ellander. Lambert had been hired to lift the curse from a duke's daughter who had been turned into the ogre and Aiden, to kill it. Things worked out badly when the ogre killed Lambert's patron and the two witchers decided to join forces.
Because the job was considered bungled, a band of trained assassins led by Jad Karadin was hired to take back the reward money from the original contract, but in the end, they killed Aiden. Karadin maintained that Aiden was killed by Vienne, but she insisted that Karadin had dealt the mortal blow.