In the Iliad, Homer plays with various foil permutations in his characterization of Agamemnon, Menelaus, Hector, and Paris. In addition to making each of the brothers foils for one another, he also creates foil dynamics across enemy lines by contrasting Agamemnon with Hector and Menelaus with Paris.
Since the reader has many different characters to stay on top of on each side of the war, it helps that the Achaean leadership and Trojan leadership are somewhat similar. Both Agamemnon—the leader of the Achaean forces—and Hector—the leader of the Trojan forces—have younger brothers. In fact, it's a dispute between these two younger brothers, Menelaus and Paris, that sparks the Trojan War. Homer characterizes both Agamemnon and Hector as stronger than their brothers. In addition, both characters express resentment towards their brothers. Through this, Homer creates a number of foil dynamics: not only Agamemnon vs. Hector and Menelaus vs. Paris, but also Agamemnon vs. Menelaus and Hector vs. Paris.
Within these brother relationships, Homer characterizes the older brother as strong and active and the younger brother as weak and passive. This difference is something the brothers themselves are aware of. Multiple times in the poem, both Agamemnon and Hector express frustration over their brothers. When Paris recoils from battle in Book 3, Homer describes Hector "[raking] his brother with insults, stinging taunts":
Paris, appalling Paris! Our prince of beauty—
mad for women, you lure them all to ruin!
Would to god you’d never been born, died unwed.
That’s all I’d ask. Better that way by far
than to have you strutting here, an outrage—
a mockery in the eyes of all our enemies.
While Menelaus is stronger and more active than Paris, Hector's criticism of Paris comes to mind in Agamemnon's descriptions of Menelaus. In Book 10, for example, Agamemnon tells Nestor that Menelaus often "hangs back, with no heart for the work." While he acknowledges that Menelaus doesn't shrink "from action" and isn't "skittish or off guard," he makes note of Menelaus's tendency to look to him, Agamemnon, "waiting for me to make the first move."
In the Agamemnon vs. Hector foil pair, both are characterized as strong leaders and fierce fighters. However, while Agamemnon appears arrogant, selfish, and reckless, Hector appears selfless, civilized, and devoted to his family. The narrative opens with Agamemnon creating lasting problems by acting in his own self-interest. By contrast, Hector acts in the interest of his city and people throughout the narrative.
While both Menelaus and Paris are characterized by their passivity relative to their older brothers, Menelaus is a much fiercer fighter than Paris. This contrast especially comes to the surface in their confrontation in Book 3, when Paris shrinks from battle. Again and again, Homer emphasizes Paris's good looks and charm, suggesting that his attractiveness precludes any real show of strength or bravery.