Satan is the adversary of God and humanity.
The great antagonist of the Bible is a being known as Satan and the devil.
Interestingly, the Bible never tells us Satan's name. Both "Satan" and "the devil" are descriptive terms (titles of sorts). Satan means accuser or adversary. The devil means a slanderer.
(Note: Many believe the devil's name is Lucifer based on one Bible passage. I do not think that the passage refers to Satan. We'll address that later.)
Satan is one of the heavenly creatures that GOD made.
6 Now there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them.
(Job 1:6 NASB)
Like all the heavenly creatures, GOD assigned Satan a role of authority within creation.
At some point after the creation was completed, Satan developed a prideful heart and invented sin.
In rebuking the hard-hearted Jewish religious leaders of the first century, Jesus said regarding Satan:
44 You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies.
(John 8:44 NIV)
In time, desiring something greater than his assigned role, Satan left his position and set out to oppose GOD.
Satan invented lying by deceiving Eve, telling her that she would not die by eating the forbidden fruit in the Garden of Eden, instead offering her the opportunity to become like a god (a heavenly creature). Heavenly creatures, including angels, were able to be in the direct presence of GOD, and they know much more than humans.
And heavenly creatures knew one another. Heavenly creatures knew Satan, the inventor of evil. Heavenly creatures knew both good and evil.
Enticed by this opportunity to become something greater, Eve bit. Literally. And Adam foolishly and weakly followed rather than interceding.
Sin, once confined to the heavenly realm, now entered the earthly realm as well.
Satan became humanity's accuser, continually bringing our guilt and unrighteousness before GOD. And throughout the Bible story, Satan continued his efforts to spread evil, destroy human lives and hurt GOD as deeply as possible. And he wasn't alone. Many joined his forces and fought on the side of evil.