The afterguard is a stranger that appears on deck to ask Billy if he will take part in a mutiny. He tells them that there is a group of discontent men, offers him some gold coins, and asks, "Couldn't – you – help – at a pinch?" (14.7). Billy becomes angry and the afterguard scampers off. When the other foretopmen arrive on the scene, Billy doesn't want to start pointing fingers so he simply says that an afterguard was out of position and he yelled at him.
The afterguard's behavior perplexes Billy. No less because a few days after the initial incident, the afterguard passes Billy and lets out "a flying word of good-fellowship" (15.4). It is strongly implied that the afterguard's encounter with Billy is part of Claggart's design, but the narration leaves it ambiguous.