The spy in The Killer Angels, like such men everywhere, is a man whose loyalty to any cause is ambiguous or non-existent. Longstreet does not trust him, though he's indispensable because the Confederates need him to reveal the position of the Union army. But the man could as easily be seen as a double agent, someone intentionally misleading Longstreet as part of a ruse to lead the Confederates into a trap. Longstreet ends up believing him based largely on his own instincts. But the spy faces the danger of being caught and executed by the Union forces or suffering the same fate at the hands of the Confederates if his information proves false. Unlike ordinary combatants, spies were given no quarter and were subject to the death penalty.
When Longstreet takes the man, Harrison, to see General Lee, the spy (or "scout") is predictably fawning and "worshipful." This attests to the exalted status Lee had acquired in only a single year as commander of the Army of Northern Virginia—or it simply shows the spy as a sycophant. He even speaks to Lee in French (which Longstreet notes is a kind of strange, "southern" French). The deferential way the spy acts isn't surprising given his uncertain position among the army officers.
The greater significance of this episode, I believe, is to show the random nature of war and the fact that the other side's troop movements are often a matter of guesswork or chance information. We also see a contrast between Harrison's seediness and the honorable qualities of the officers on both sides of the conflict, as will be shown us throughout the novel as it progresses through the battle of Gettysburg.