As Achilles’s beloved companion and advisor, Patroclus’s primary role within The Iliad is to support Achilles. The story hinges on Achilles’s choices and emotions, including his affection for Patroclus. Nestor appeals to Patroclus specifically to attempt to convince Achilles to regain the battle against the Trojans, and it is only Patroclus’s death which finally does so.
Patroclus is given little attention until Book Nine. When the Achaean envoy approaches Achilles to persuade him to rejoin the battle, they find him alone in his tent with Patroclus. Achilles is singing and playing the harp, and Patroclus seems happy to simply sit and listen. Then, when Achilles invites the Achaeans into his tent, Patroclus takes on most of the labor of hospitality. He tends the food and fire while Achilles swaps stories with their guests. Patroclus, rather than Achilles, directs the servants to prepare a bed for Phoenix. Although Achilles is technically the host, it seems Patroclus has enough of a position in Achilles’s household to tend to the needs of Achilles’s guests.
Patroclus proves again that he is capable of deep care and service for his friends when he tends to Eurypylus’s wound. The horrors of the war overwhelm him, and he returns to Achilles’s tent weeping for his countrymen. When his concern finally spurs him to the battlefield, Patroclus proves an adequate warrior, but he pushes beyond his limits and is no match for Hector.
After his death, Patroclus’s body becomes the rallying point for the Achaean army. They fight against unreasonable odds to protect his body for Achilles’s sake. When Achilles himself comes to take vengeance for Patroclus’s death, he singlehandedly turns the tide of the war in his rage. Without Patroclus’s death, Achilles would not overcome his anger at Agamemnon, and the Achaeans would lose the war. We hear about Achilles’s devotion to Patroclus in life, but we see it fully demonstrated after Patroclus’s death.