Patroclus, son of Menoetius, was Achilles' devoted friend and companion. As a boy, he'd accidentally killed a playfellow and been forced to go on the run with his father. Peleus, father of Achilles, took them in and brought Patroclus up as one of the family. He was older than Achilles, and thus feels a responsiblity towards him. He withdraws from the fighting when Achilles does in Book 1, only returning in Book 16, wearing Achilles' armour.