Flagg was a large male otter from Redwall Abbey that was one of the main leaders of the defense of Redwall against the crew of Greypatch. He also discovered Greypatch's crew outside the Abbey and overheard their plan of attack when he had gone out to go after Mariel and her companions on Mellus' request. He carried a sling and stones, and helped Mother Mellus save the life of Hon Rosie.