Ernie is the gruff, quiet driver of the Knight Bus. And he probably needs to re-take his driver's license exam, given how he mans the Knight Bus. Though he's quiet, Ernie serves as a sort of emotional barometer of wizarding community via his reactions to the overly-talkative Stan. When Ernie does speak, it's worth noting, and what he says tells us a lot about how the wizarding community in general feels about certain subjects, namely Sirius Black and Voldemort. While Stan relishes the story of Black with the glee of someone who would enjoy reading the tabloids, Ernie finds the topic disturbing and wants to stop discussing it. Through Ernie, we get a hint of how Black's crime impacted the community and added to the fear instilled there by Voldemort. Ernie serves as a nice way to reintroduce Harry to the wizarding community, and his reluctance to discuss dark times of the past sets the tone for many of the novel's upcoming scenes, particularly ones with adults who get noticeably close-lipped around kids like Harry.