Alfred. I was watching from season 2 reruns of Arrow, and I was thinking that he is Oliver’s common sense when everything starts to fall apart. I know, Felicity is his heart but Diggle was there from the beginning, even before she arrived on Team Arrow. Diggle can relate to Oliver due to their field experiences and he was the first to let Oliver know that he would question his choices from the moment they started the crusade. Diggle and Oliver act more like brothers than friends and I think Oliver relies on him to keep him from going over the edge, just like Alfred does for Batman.