Betteredge is a strange character, in the sense that he is entirely defined by others and not himself. He performs very few actions, and spends a large amount of his given time describing his wife, daughter, lady Verinder, the family he serves and most importantly Robinson Crusoe. What most defines him is the repeated action of a re-reading the same book, almost as if it was a personal bible. An interesting side note would be to graph how much of his descriptions are of those around him.
While the first narrator, invokes, truth, honor, and swears of his duty as a soldier, Betteredge's honesty can be ascertained as believable through his lack of self, his humbleness and plain character in which he prefers to describe a world inhabited by others. Certainly that is a man to be trusted in describing a following fantastic tale. Collins seeks to reassure the reader that the narrators are trustworthy