Satan, formerly known as the archangel Lucifer in Heaven, is cast into Hell after waging a battle against God with the other fallen angels he has convinced to join his rebellion. At times conflicted about his intentions in the face of an all-powerful God, he ultimately realizes he is doomed to suffer eternally. He attempts to corrupt God's newly created race of humans by tempting the first two humans, Adam and Eve. Even though Satan is the evil antagonist of the story, he is also the most complex character, and his perspective is compelling.
God is the ruler of Heaven and the universe. He sees and knows everything, and, though he expects obedience from all that he has created, he has also endowed all creatures with free will so that their love and obedience is by choice. Even though Satan rebels against him, God already knows the outcome of the rebellion and Satan's corruption of Adam and Eve. God is not very relatable; he does not have emotions like the other characters, and his pronouncements often seem cold and harsh.
The Son is difficult to conceptualize, because he is essentially the same as God, just a different part of him. His role in Paradise Lost is more a role of action, as he volunteers to die for the sins of mankind. After he dies, he returns to life as Jesus to defeat Satan before returning to Heaven.
Adam is created by God to be innocent and perfect. Even though God knows Adam's past, present, and future, he imbues him with free will, which causes Adam to be susceptible to temptation. Adam is closer to God than Eve and is supposed to be smarter and stronger. He is curious about his surroundings and asks Raphael questions about the astronomical workings of the universe and whether angels eat food, showing that humans are innately curious and desire knowledge.
Eve is Adam's companion, and Raphael tells Adam that she is lesser than him in wisdom and strength, although Adam considers her as wise and as equal to him. Satan chooses to tempt her, because he sees her as an easier target, and convinces her to eat the fruit, leading to the fall of man.