This is a classic literary debate, and a strong case can be made for why readers, especially from the Romantic period, saw Satan as the hero. However, within the value system Milton intentionally constructed for the poem, the Son of God is the ultimate hero.
The Case for Satan (The "Apparent" or "Romantic" Hero)
For the first two books, Satan certainly fits the mold of a classical epic hero. He is a powerful leader who is defiant in the face of overwhelming odds, rallies his defeated troops with charismatic speeches, and undertakes a perilous journey alone for the sake of his people. The poet William Blake famously wrote that Milton was "of the Devil's party without knowing it," because Satan is by far the most dynamic and compelling character in the early parts of the epic.
The Case Against Satan
However, Milton systematically dismantles Satan's heroic stature throughout the poem. His motivations are not noble; they are rooted in pride, envy, and a thirst for revenge. His "heroic" quest is not to build or create, but to corrupt and destroy God's innocent creation. His character degrades over the course of the epic: he begins as a magnificent fallen archangel (Book I), but by the end, he is reduced to a hissing, groveling serpent, punished by being turned into his own disguise (Book X).
The Case for the Son of God (The "True" Hero)
Milton's epic redefines heroism, moving it away from the classical model of defiant strength and toward a Christian model of obedience, sacrifice, and redemptive love.
The Ultimate Heroic Act: The central heroic moment in the poem occurs in Book III, long before Adam and Eve even fall. When God the Father asks who will save mankind from the death they will bring upon themselves, there is silence in Heaven. It is the Son who freely offers himself as a sacrifice, saying, "Behold me then, me for him, life for life / I offer." This act of selfless love and willing sacrifice is presented as far more heroic than Satan's prideful rebellion.
True Power and Victory: The Son is also the one who single-handedly defeats Satan's armies in the War in Heaven (Book VI). While Satan gains a temporary victory by corrupting mankind, it is the Son's future incarnation and sacrifice that will fulfill the prophecy of "bruising the Serpent's head" and ensuring humanity's ultimate redemption.
While Adam is the central human protagonist, he is a fallen figure whose heroism is found in his repentance, not his actions. The Son of God is the only character who acts with perfect obedience, selfless power, and redemptive purpose, making him the true hero of Milton's theological vision.