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Paradise Lost's Satan is perhaps one of the most interesting and complex character to ever be written. He is presented as a vain, yet extremely charismatic leader. Before he led the revolt in heaven, and was thrown down into hell, he was known as Lucifer. Lucifer was an archangel, one of God's most trusted and highest ranking angels. It was perhaps the ultimate betrayal to have Lucifer lead a rebellion.
The name "Satan" literally means "enemy" in Hebrew. Satan is described as being physically immense, and in the beginning of the poem he still possesses his famed good looks, however, throughout the story we see Satan degenerate into a hideous shell of his former self. Readers can see this particularly after the fall of Man. Milton has frequently been critiqued for his sympathetic description of Satan. It is sometimes speculated that Milton built himself into Satan's character. Throughout the entirety of the piece Satan refuses to surrender his beliefs regarding his right to independence. He vows revenge against God, and refuses to acknowledge that God was the creator of all, and this, most people believe, is his greatest downfall.
Satan is regularly thought to be an excellent example of an epic hero. The language that Milton uses for Satan's speeches and lines are particularly moving, making it incredibly obvious how 1/3 of the angels in heaven would follow him to hell. It's important to note how attractive Satan makes being evil seem, he demonstrates that there's an intense freedom that comes with being evil. One can almost understand Satan's point of view, which makes him all the more dangerous as a character.