Killconey is a ferret from Mossflower Woods who joined the horde of Cluny the Scourge when Cluny attacked Redwall Abbey. He was an extremely talkative ferret with a comical accent (probably Irish) and a tendency to refer to his "ould" mother's sayings. But he wasn't a fool. In fact, he turned out to be very useful for Cluny in several schemes.
Killconey and a rat named Cheesethief were the only members of Cluny's horde who ever saw the adder Asmodeus Poisonteeth and lived; they saw him pulling the carcass of Ragear away. He was later in charge of the crew of weasels, stoats and ferrets with the duty of digging a tunnel into Redwall from the southwestern corner (though this plan was discovered and failed miserably, leading only to the death of Darkclaw and many other hordesbeasts). During the largest battle of the Late Rose Summer Wars, Killconey for a short while took the position of "drummer-cum-soothsayer".
Also, he had some knowledge of leverage, and built Cluny a siege tower to get into Redwall from the back wall, though this scheme also failed to gain access into the Abbey. The incident shocked Cluny so much that for a short while, Killconey took control of the horde along with Darkclaw and Fangburn. In the final battle for Redwall, Killconey was forced by Cluny to be a living shield and thus was slain by a swipe from Matthias that chopped him in half. His final sacrifice saved his Chief's life, if only for a short while.
While never shown in a positive light, Killconey is one of the least evil villains in Redwall franchise. This may be linked to his being forced into service under Cluny.