Sergeant Hobo 678 is the leader of Zim's training session on Hobo 13. Sergeant Hobo 678 says that the team needs a leader, and Zim votes himself and wins. Zim then goes on to get all his team mates eliminated, to the extreme disgust of Sergeant Hobo 678.
Sergeant Hobo 678 is voiced by the legendary R. Lee Ermey, a former drill sergeant famous for his role as Gunnery Sergeant Hartman in the film Full Metal Jacket. Since then, he has spent much of his acting career playing similar characters, many of them parodies of his "hard-edged drill sergeant" image.