This is a complex question. The title of the poem does not give us any guidance, either. The Aeneid, for example, is named after Aeneas, literally the story of Aeneas, which helps us to understand quickly that Aeneas is the intended hero and protagonist. The Iliad means simply "the story of Troy" and, as such, we are invited to consider the Trojan Wars from the perspective of either side, or both.
Arguably, then, each side is furnished with its own hero. Achilles is the great hero of the Achaeans, and the story revolves around his disagreement with Agamemnon, who has caused the trouble by taking Chryseis captive. So, interestingly, modern readers would tend to perceive Agamemnon as the villain of the piece, even though he is technically on the same side as Achilles. Meanwhile, Hector is the great hero of the Trojan side, posited as an equal to Achilles, who is more than human.
There are other characters on both sides, however, who might be called heroes, such as Paris, Ajax, and Patroclus....
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