Charlie and Eli are brothers. Patrick DeWitt does a wonderful job of portraying them as brothers that are extremely close. They are not exactly a brotherly pair with high moral standings. They are professional hit men. Despite their odd profession, they are good at it, because they are so close. Throughout the novel, DeWitt lets his readers know that Charlie and Eli are capable of having entire conversations about events simply by having a look at each other.
Their relationship isn't perfect and completely happy though. Eli is the narrator of the story, and his descriptions of his brother are not always flattering, but it is clear that Eli respects Charlie's intelligence and logic.
The two brothers may not outwardly tell each other that they "love each other like brothers," but it is clear in how they stick together through thick and thin that their familial bond is very powerful.