Satan (sometimes known as "Lucifer") is a recurring character in the anthropomorphic webcomic Jack, although he is normally confined to the deepest region of Hell he has shown himself on varied occassions: both in his current state as the "Lord of Darkness" and in his former life as a powerful angel in the presence of God.
According to the somewhat unusual cosmology of Jack's universe, Satan was once one of God's most beloved creations and went by the name of Lucifer, he was the one who suggested to God the act of creation, which eventually led to existence itself.
However, Lucifer eventually rebelled against God, waging war with many of his fellow angels. However, he lost the war, and was banished from Heaven, residing in Hell.
Now residing at the very bottom of Hell, Satan works to corrupt and tempt the living, to lead them astray and into the ways of evil, away from God.
Though Satan is the supreme force of evil, he seems to take little interest in Hell itself, or it's occupants, even stating in a hidden comic that Hell will one day have a new king, and that he can have it, implying that Satan doesn't care about ruling Hell at all, though he is not above interacting with it, as Satan has directly punished two souls personally, and has talked with both Jack and Drip directly (though the interaction with Drip took place off screen).
Although Satan is a fallen angel he still maintains a vast array of supernatural powers such as the ability to change form or gender and can even temporarily travel to Heaven or Earth (though he must always ultimately return to Hell).
Unlike many depictions of Satan, it is also hinted that in Jack's universe at least even Satan may have a chance for redemption if he apologizes for what he has done - however it seems unlikely that a being like Satan will ever truly accept salvation and for now at least he remains the "Prince of Darkness".