Satan sought to make himself equal to God. He said
"I will ascend above the heights of the clouds,
I will be like the Most High." Is. 14:14
For that reason, God brought him down. He still has access to heaven, at least for the time being. This doctrine is derived from Job 1:6.
"Now there was a day when the sons of God came to present themselves before the LORD, and Satan also came among them."
I'm not sure "what is his role?" is the right way to word the question as it implies that God has us all involved in some kind of a RPG, which is not the case.
A better way to word the question is "what is his mission or goal?"
In a nutshell, Satan works to:
1) pervert the scriptures, twisting them to say what they do not mean.
Mt. 4:6
and said to Him, If You are the Son of God, throw Yourself down. For it is written: He shall give His angels charge over you,
and, In their hands they shall bear you up,
Lest you dash your foot against a stone.
Satan knew that God would protect His Son, and tempted Jesus to do something which would reveal himself as the Son of God prematurely. Jesus knew that God had promised to preserve Him, but that that guarantee presupposed living in God's will. To claim the promise in an act of disobedience would be tempting God. Jesus chose to await God's timing to accomplish His will.
Gen. 3:1-5
Now the serpent was more cunning than any beast of the field which the LORD God had made. And he said to the woman, Has God indeed said, You shall not eat of every tree of the garden?
2 And the woman said to the serpent, We may eat the fruit of the trees of the garden; 3 but of the fruit of the tree which is in the midst of the garden, God has said, You shall not eat it, nor shall you touch it, lest you die.
4 Then the serpent said to the woman, You will not surely die. 5 For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.
Satan knew that God did not mean she would die physically right away, but that disobedience would break the bond she enjoyed with God. He twisted what God said to imply that she would not die physically from a "poison" fruit. Eve sinned, and the relationship with God was broken, resulting in spiritual death, physical death to come later.
2) he opposes God's work.
Zech. 3:1
Then he showed me Joshua the high priest standing before the Angel of the LORD, and Satan standing at his right hand to oppose him.
1 Thes. 2:18
Therefore we wanted to come to youeven I, Paul, time and againbut Satan hindered us.
3) he hinders the gospel.
Mt. 13:19
When anyone hears the word of the kingdom, and does not understand it, then the wicked one comes and snatches away what was sown in his heart.
2 Cor. 4:4
whose minds the god of this age has blinded, who do not believe, lest the light of the gospel of the glory of Christ, who is the image of God, should shine on them.
4) he works lying wonders.
2 Thes. 2:9
The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders,
Rev. 16:14
For they are spirits of demons, performing signs, which go out to the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.
5) he appears as an angel of light.
2 Cor. 11:14
And no wonder! For Satan himself transforms himself into an angel of light. 15 Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, whose end will be according to their works.
He makes sinful things look appealing, even right to men, but such things lead to destruction.
Prov. 14:12
There is a way that seems right to a man,
But its end is the way of death.
6) he is the father of lies.
John 8:44
You are of your father the devil, and the desires of your father you want to do. He was a murderer from the beginning, and does not stand in the truth, because there is no truth in him. When he speaks a lie, he speaks from his own resources, for he is a liar and the father of it.
In all of these things and more, Satan seeks to subvert God's plan for the salvation of men. He is crafty, a deceiver, an adversary to God and the work of Godly men.
Therefore, we are warned about him.
Eph. 6:11-13
Put on the whole armor of God, that you may be able to stand against the wiles of the devil. 12 For we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this age, against spiritual hosts of wickedness in the heavenly places. 13 Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand.
1 Pet. 5:8
Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour. 9 Resist him, steadfast in the faith, knowing that the same sufferings are experienced by your brotherhood in the world.
Satan is aware of the fate that awaits him, and wishes to take as many as he can with him. But, you can be sure, through belief in Jesus Christ, that you will not find yourself on the left hand of God at the judgment.
Mt. 25:41
Then He will also say to those on the left hand, Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels