Also refered to by his God-given name of Lucifer, he is the highest angel in rebellion to God. After his attempts to be like God failed, he was cast down from Heaven as a fallen angel. His current dominion is hell.
Satan is the most evil being to ever exist. His rebellious nature against God gives him pride in taking as many souls with him to damnnation as possible. In short, he knows he's going down, so he wants to take down as many people with him as he can.