Satan is the first major character introduced in the poem. Formerly called Lucifer, he was the most beautiful of all angels in Heaven, and is a tragic figure who describes himself with the now-famous quote "Better to reign in Hell than serve in Heaven." He is introduced to Hell after he leads a failed rebellion to wrest control of Heaven from God. Satan's desire to rebel against his creator stems from his unwillingness to be subjugated by God and his Son, claiming that angels are "self-begot, self-raised," and thereby denying God's authority over them as their creator.
Satan is deeply arrogant, albeit powerful and charismatic. Satan's persuasive powers are evident throughout the book; not only is he cunning and deceptive, but he is also able to rally the fallen angels to continue in the rebellion after their agonizing defeat in the Angelic War. He argues that God rules as a tyrant and that all the angels ought to rule as gods. Though commonly understood to be the antagonizing force in Paradise Lost, Satan may be best defined as a tragic or Hellenic hero. According to William McCollom, one quality of the classical tragic hero is that he is not perfectly good and that his defeat is caused by a tragic flaw. As Satan causes both the downfall of man and the eternal damnation of his fellow fallen angels despite his dedication to his comrades, Satan is perhaps an ur-example of the trope. In addition, Satan’s Hellenic qualities, such as his immense courage and perhaps, lack of completely defined morals, compound on his tragic nature.
Satan's status as a protagonist in the epic poem, however, is debatable; Milton arguably characterizes him as such, but Satan lacks several key traits that would otherwise make him the definitive protagonist in the work. One deciding factor that insinuates his role as the protagonist in the story is that most often a protagonist is heavily characterized and far better described than the other characters, and the way the character is written is meant to make him seem more interesting or special to the reader  For that matter, Satan is both well described and is depicted as being quite versatile in that he is shown as having the capacity to do evil whilst retaining his characteristic sympathetic qualities and thus it is this complex and relatable nature makes him a likely candidate for the story’s overarching protagonist.
However, Ibrahim Taha’s definition of a protagonist states that the protagonist must be able to exist in and of his or herself and that the secondary characters in the work exist only to further the plot for the protagonist. Because Satan does not exist solely for himself, as without God he would not have a role to play in the story, he may not be viewed as protagonist because of the continual shifts in perspective and relative importance of characters in each book of the work. Satan’s existence in the story involves his rebellion against God and his determination to corrupt the beings he creates in order to create evil so that there can be a discernable balance and justice for both himself and his fallen angels. Therefore, it is more probable that he exists in order to combat God, making his status as the definitive protagonist of the work relative to each book.
Satan’s status as a traditional hero in the work is similarly up to debate as the term “hero” evokes different meanings depending on the time and the person giving the definition and is thus a matter of contention within the text. According to Aristotle, a hero is someone who is “superhuman, godlike, and divine” but is also human. A hero would have to either be a human with God-like powers or the offspring of God. While Milton gives reason to believe that Satan is superhuman, as he was originally an angel, he is anything but human. Therefore, according to Aristotle’s definition of a hero alone, Satan is not a hero. Torquato Tasso and Francesco Piccolimini expanded on Aristotle’s definition and declared that for someone to be considered heroic one has to be perfectly or overly virtuous. Satan repeatedly demonstrates a lack of virtue throughout the story as he intends to tempt God’s creations with evil in order to destroy the good God is trying to create. Satan goes against God’s law and therefore becomes corrupt and lacking of virtue and, as Piccolimini warned, “vice may be mistaken for heroic virtue”. Satan is very devoted to his cause, although that cause is evil but he strives to spin his sinister aspirations to appear as good ones. Satan achieves this end multiple times throughout the text as he riles up his band of fallen angels during his speech by deliberately telling them to do evil to explain God’s hypocrisy and again during his entreaty to Eve. He makes his intentions seem pure and positive even when they are rooted in evil and according to Steadman, this is the chief reason that readers often mistake Satan as a hero.
Though Satan's army inevitably loses the war against God, Satan achieves a position of power and begins his reign in Hell with his band of loyal followers, composed of fallen angels, which is described to be a "third of heaven". Satan's characterization as the leader of a failing cause folds into this as well and is best exemplified through his own quote, "to be weak is to be miserable; Doing or Suffering", as through shared solidarity espoused by empowering rhetoric, Satan riles up his comrades in arms and keeps them focused towards their shared goal. Similar to Milton’s republican sentiments of overthrowing the King of England for both better representation and parliamentary power, Satan argues that his shared rebellion with the fallen angels is an effort to “explain the hypocrisy of God”, and in doing so, they will be treated with the respect and acknowledgement that they deserve. As scholar Wayne Rebhorn argues, “Satan insists that he and his fellow revolutionaries held their places by right and even leading him to claim that they were self-created and self-sustained” and thus Satan’s position in the rebellion is much like that of his own real world creator.
However, the true nature of his role in the poem has been the subject of much notoriety and scholarly debate. While some scholars, like the critic and writer C. S. Lewis, interpret the poem as a genuine Christian morality tale, other critics, like William Empson, view it as a more ambiguous work, with Milton's complex characterization of Satan playing a large part in that perceived ambiguity.
Adam is the first human created by God. Though initially alone, Adam demands a mate from God. Considered God's prized creation, Adam, along with his wife, rules over all the creatures of the world and resides in the Garden of Eden. He is more gregarious than Eve, and yearns for her company. His complete infatuation with Eve, while pure in and of itself, eventually contributes to his joining her in disobedience to God.
Unlike the Biblical Adam, before he leaves Paradise this version of Adam is given a glimpse of the future of mankind (including a synopsis of stories from the Old and New Testaments) by the Archangel Michael.
Eve is the second human created by God, taken from one of Adam's ribs and shaped into a female form of Adam. Far from the traditional model of a good wife, she is often unwilling to be submissive towards Adam. She is more intelligent and curious about external ideas than her husband. Though happy, she longs for knowledge and, more specifically, self-knowledge. Her first act in existence is to turn away from Adam and look at and ponder her own reflection. Eve is extremely beautiful and thoroughly in love with Adam, though may feel suffocated by his constant presence. One day, she convinces Adam that it would be good for them to split up and work different parts of the Garden. In her solitude, she is tempted by Satan to sin against God. Adam shortly follows along with her.
The Son of God
The Son of God is the spirit that will become Jesus Christ, though he is never named explicitly, since he has not yet entered human form. The Son of God shares total union with God, and indeed is understood to be a person of the Godhead, along with the Father and the Spirit. He is the ultimate hero of the epic and is infinitely powerful, singlehandedly defeating Satan and his followers and driving them into Hell. The Son of God tells Adam and Eve about God's judgment after their sin. However, he sacrificially volunteers to eventually journey to the World, become a man himself, and redeem the Fall of Man through his own death and resurrection. In the final scene, a vision of Salvation through the Son of God is revealed to Adam by Michael. Still, the name, Jesus of Nazareth, and the details of Jesus' story are not depicted in the poem.
God the Father
God the Father is the creator of Heaven, Hell, the world, and of everyone and everything there is. He desires glory and praise from all his creations. He is an all-powerful, all-knowing, infinitely good being who cannot be overthrown by even the great army of angels Satan incites against him. The stated purpose of the poem is to justify the ways of God to men, so God often converses with the Son of God concerning his plans and reveals his motives regarding his actions. The poem portrays God's process of creation in the way that Milton believed it was done, with God creating Heaven, Earth, Hell, and all the creatures that inhabit these separate planes from part of Himself, not out of nothing. Thus, according to Milton, the ultimate authority of God derives from his being the "author" of creation. Satan tries to justify his rebellion by denying this aspect of God and claiming self-creation, but he admits to himself this is not the case, and that God "deserved no such return/ From me, whom He created what I was."
Raphael is an archangel whom God sends to warn Adam about Satan's infiltration of Eden and to warn him that Satan is going to try to curse Adam and Eve. He also has a lengthy discussion with the curious Adam regarding creation and events which transpired in Heaven.
Michael is a mighty archangel who fought for God in the Angelic War. In the first battle, he wounds Satan terribly with a powerful sword that God designed to even cut through the substance of angels. After Adam and Eve disobey God by eating from the Tree of Knowledge, God sends the angel Michael to visit Adam and Eve. His duty is to escort Adam and Eve out of Paradise. Before he does this, Michael shows Adam visions of the future which cover an outline of the Bible, from the story of Cain and Abel in Genesis, up through the story of Jesus Christ in the New Testament.