This article identifies and defines a recurrent figure in Sophoclean tragedy, which I call the co-operator, distinct from the more familiar hero and adviser figures. This figure is distinguished primarily by his role (sic: the figure is typically male), which is to be someone on whom the hero must depend. He acts without obligation and frequently against expectation. This behaviour can therefore only be explained in terms of the character of the co-operator: a propensity to respect and sympathise with the hero. I use the term 'role' to mean the dramatic function assumed by a figure within the plot. A character can assume a role, or discard it, at various stages in a play. Hence, not all the co-operators begin as co-operators. An extreme example of a drama in which roles can be fluid is Antigone, which I discuss towards the end of the article. Significant variation on the theme of the co-operator is also found in the role of Hyllus in the exodos of Trachiniae.