What is the relationship between Artemis and Butler?

Butler is Artemis's bodyguard and has been doing so all of Artemis's life. But beyond guarding the boy, Butler has become Artemis's father figure. Artemis is Butler's closest friend. The relationship between the two becomes more complex in that they are about to embark in criminal activities. Since Butler is the adult, he should be the voice of reason. However, Artemis is the mastermind, so Butler seems to do as he is told. As pointed out by the author, "This is no ordinary relationship."