Diomedes (also called “Tydides,” meaning “son of Tydeus”) is the youngest of the Achaean commanders. While Diomedes may not be a demigod like Achilles, he is a bold and talented fighter who emerges to fill the void when Achilles refuses to fight. He receives divine assistance in the form of Athena who gifts him with superhuman strength and the ability to identify gods on the battlefield. He becomes so powerful that he is able to wound two gods, Aphrodite and Apollo, rendering him an invaluable member of the Achaean forces.
Over the course of the epic, it becomes clear that Diomedes serves as Achilles’s quasi-replacement. Homer states as much in Book 5 when Helenus the seer declares that “[Diomedes] is the strongest Argive” now that Achilles has stopped fighting. Homer also juxtaposes Achilles’s petulance and his unwillingness to fight with moments in Books 9 and 14 in which Diomedes pushes back against the Greeks, despite their wish to surrender. Diomedes serves as something of a foil to Achilles—that is, Diomedes’ unwavering commitment to the Greek cause highlights Achilles’s stubbornness. In essence, he might be considered Achilles without the ego.
It should be noted that, despite his many talents, Diomedes can never truly replace Achilles; no man can. Homer explicitly says so when Hera observes that “as long as brilliant Achilles stalked the front, no Trojan would ever venture beyond the Dardan [Trojan] Gates.” In Book 11, Diomedes is injured in battle and effectively sidelined, just in time for Achilles to return and reclaim his status as the best of the Greeks. Diomedes is injured when Paris strikes him in the foot with an arrow; notably, Achilles too will ultimately fall victim, though in his case fatally, to an arrow to the heel shot by Paris.