Character Role Analysis
Although he is older than Achilleus and was instructed by his father to give the brash warrior advice, Patroklos is not much of a mentor figure. He mainly plays the role of Achilleus's best friend, accompanying him on his campaigns and hanging out with him when he's playing the lyre. He even gets stuck performing some minor chores for Achilleus, such as checking on the identity of Machaon.
At the same time, Patroklos acts as Achilleus's conscience. We get some sense of this from Briseis's speech in Book 19, when she tells us that Patroklos promised to convince Achilleus to take her as his lawful wife. More importantly, he speaks up on behalf of the other Achaians in Book 16. This leads to his brave actions in leading the Myrmidons into battle, but also in his death at the hands of Apollo, Euphorbos, and Hektor. Ultimately Patroklos is most important for what he means to Achilleus. Grief at the loss of his friend is Achilleus's main motivation in the final books of the Iliad.