Since Paradise Lost is an epic poem that retells the Bible account of creation and beyond, the hero is Jesus, the son of God. Everything is fine in heaven until Satan, prideful, challenges God and is thrown out of heaven. From that point on, Satan and his demons make several plans to regain entry into heaven, but fail. So, they turn their attention to the world, knowing that created man is weak and will be much easier to conquer than God and his angels. Satan's spirit enters the sleeping serpent, and the serpent tempts Eve to eat fruit from the forbidden tree. He tells Eve that God has not told her and Adam the entire truth, and surely they will not die if they eat this fruit. They are tempted, they eat, and then sin enters into Paradise. Adam and Eve are first lustful then shameful. They hide from God, but eventually, they are expelled from the Garden of Eden. Luckily, the Son has offered his own life to redeem fallen mankind and offers to die for the sins of the world. Thus, God gives grace to mankind for their sins, even though they don't deserve it, and they have eternal life through his Son, who has sacrificed his life for the sins of mankind. So Jesus is the hero.
I would argue that there are two heroes in Milton's work. The first hero is Satan. He is the poem's portrayal of the epic hero. Satan is Milton's answer to the Vergillian and Homeric heroes in that he is war-like and valiant (like Achilles), cunning and guileful (like Odysseus), and inspiring and commanding (like Aeneas); at once the archangel is the warrior, the tactician, and the leader.
Satan is also one of the three characters in the poem with the Aristolean hamartia (along with Adam and Eve). His flaw, or at least his most recognizable flaw, is pride. Satan aspires to equal God, renouncing the latter as his creator and labelling Him a tyrant. However, it is proven that Satan's might is no match for the thunder of the Son, and he is hurled from heaven to live in the lake of fire for eternity.
Later, Satan escapes the torments of hell, and sets off on an immense journey through Chaos. This journey smacks of the journeys each ancient epic hero undergoes (Odysseus' journey back to Ithaca, the Achaen's journey and warring at Troy, and Aeneas' journey to found Rome).
Satan is also the only character in the poem with a clear, definable goal: to harm God by corrupting Man. This is yet another trait that is paralleled in the three ancient epics.
The second hero I've spoken of would be Milton, himself. While this may seem odd at first, one must look at how the poet is expressed in the poem. Abdiel, Michael, Raphael, the Son, and even Satan all can be seen as a portrayal of Milton. Abdiel being the pious and devout Milton who stands for what is correct and virtuous, even when alone. Michael and Raphael can represent the protector Milton -- fighting off evil with the knowledge the poet imparts unto the public in his various tracts. The Son perhaps represents the holy knowledge that Milton feels he's attained, having been supposedly visited by the heavenly spirits during the writing of Paradise Lost. Finally, Satan is a representation of the freedom-fighting Milton. In Areopagitica, Milton makes many of the same arguments Satan makes (such as the necessary knowledge of both good and evil in order to know and choose good). However, Satan can also represent what Milton would become if he were to lose sight of God. Only with God's aid can freedom truly be won, and Milton reminds himself here with Satan.