A serpent,  the evil one, satan, the devil, lucifer, the prince of the power of the air, and so on, etc.  There is no point in dignifying those names with a capital letter.  We know who "it" is.

I have a friend who teaches teenagers and tells them, "Sin is so momentarily fun.  If it wasn't, it wouldn't be any temptation at all.  But the payoff will kill you."   Satan knows what is tempting to you.  He knows all of our personal weaknesses--and that is what he will throw at you every time.  Over and over again, you will be tempted by the same things.  It will be the same old fight every day.  And without the In-dwelling Spirit, you will lose the fight.  But by resisting, you do get stronger.  Likewise, by giving in, you get weaker.  Then it gets easier and easier to give in.  Before you know it, you are changed into a person you never wanted to be.  It's a spiritual disease called "Hardening of the Heart."

Where did this horrible evil-spirit creature come from that is intent on destroying people's lives?  And why!!?  He slithered out of Eden and has been intent on human destruction ever since.  Why is he here in the first place.  Well, truth is, I don't know.  All I can do is quote scripture.  And the problem with that is that the scripture on the subject would fill this page.  So I will pick out a few thoughts.

Is. 14:12-13a  "How you are fallen from heaven, O Lucifer, son of the morning!...For you said to yourself, "I will ascend to the heaven and rule the angels..." (He thought he was equal with God.)

First, people are not the only created beings.  God created angels as well.  Lucifer decided to rule the angels.  God punished him, cast him out of heaven, banished him to earth, and in Is. 14:16b-17 after he was cast out, God said to him, ...Can this be the one who shook the earth and kingdoms of the world?  Can this be the one who destroyed the world and made it into a shambles, who demolished its great cities and had no mercy,,,"

Sounds like the condition of the world that God had created perfect--that was destroyed--somewhere between Gen.1:1 and Gen. 1:2.  A condition that was dark, void and without form.