This is a fascinating question to consider, because in many ways there is no one character who stands out clearly as a hero in this text. Rather, Milton seems to present Satan as a kind of anti-hero, who is the protagonist but who definitely does not display heroic qualities. This is something that critics have grappled with since Milton wrote this epic classic, because Milton ostensibly wrote an epic about God to justify God's ways to man, yet in that epic he presents Satan in a very sympathetic light. This is partly because he resembles humans so much in his inconsistency and faults. Satan in many ways acts as a character who embodies the various failings of humans, and it is his frequent inconsistencies that cause the audience to recognise their own errors. It is much easier to relate to this all-to-human character than it is to relate to God or Jesus, who are, by their very definition, without sin. Note how Satan displays his error in the following speech he makes to Eve when he tries to tempt her:
Look on me!
Me who have touched and tasted yet both live
And life more perfect have attained than fate
Meant me, by vent'ring higher than my Lot.
Of course, it is impossible to live a "more perfect" life than a life that is already perfect. Such inconsistencies reveal the illogical nature of Satan's argument whilst also indicating the massive delusion pride causes on his way of viewing the world. Such failings cause him to be an immensely sympathetic character, whilst at the same time ensuring that the audience never loses focus on his evil nature and reprehensible actions. This epic does not therefore contain a hero, but an anti-hero in the form of Satan.