Hmm depends...Satan has different roles in different religions.
Lets just imagine for Christianity; what if there was no Satan in there? How would peeps find closure in explaining why mankind does bad things. The the psychological value of Satan's existence.

As for who he really is and whether he's real...I honestly don't know.

Literally is my left buttcheek.

Satan means adversary and its a title taken also known as Ha Satan. As God himself, he wants us to have testing and trails, and Satan's job is to do that.  His job is pretty close to an extreme Version of a prosecutor. This is also the reason for Job's story.

For example, Jesus called Simon Peter Satan, but he wasn't reffering that Simon was Satan, rather he was just obstructing his way, rather then challenging him.

Normally though, sometimes as human we don't like simply leaving it as his job and his name sake to be the opposser. Its better that Satan has some sort of an agenda. This is where Christians like the idea that Satan is synonomous with Lucifer, and that he is fallen angel. The story comes from Ezeikiel. It leave a grounding agenda that he believes to take everyone with him as an act of revenge. The story goes, Lucifer was angel working closest to God, and very high on the heavenly chair, and leader of the choir.  But  he became so prideful that he though he was God and angels chose to worship him. Because of this prideful wickness, he threw Satan out of  Heaven and along with the 1/3 of the heaven's angels.

I actually found some doubt that Lucifer many not be Satan, as light bearer or Morning star is what Lucifer, means (hence the character Mike Morning star turning into a villian in Ben 10 as a fun fact). That name is also attributed to Jesus,  Adam, and many other people in the bible . Also that story is attributed to King Babylon who is rebelling against God.  Alot of the ideas are further transform because of fictional works, like Dante's Divine Comedy.

But, this debate about who are Satan, Lucifer, and Devil, goes into huge long history of Linguistics and cultural uses of the term, so you probably have to do more research as my post would be very very TLDR. Its a mental gymnastics to find all the exact sources. Or have Debate.org do it for you to understand all the context where Christianity get their ideas from:
www.debate.org/debates/Lucifer…


However, its also my argument, its all we got for theories of the devil. We don't know alot about Spiritual Beings, and what they do. I think even though the story of lucifer may or may not be true, it still is a possible agenda for Satan himself, even though it is his job to be testing and tribulations. People can have jobs, but people can believe in their own agendas and  religions while doing their jobs and so, I think the story can be still likely, even if Satan wasn't Lucifer.  It makes prosecutors sound eviler and his test even more annoying to beat. And just thinking of how the evilest villians work in many fictional stories, Lucy's story really suffices. 

imo

Satan is purely created for human excuses....when human do sin action, human will say "that's Satan who trick us".....and I find it terrible and if Satan is exist and purely evil I will not say he is bad.....imo.....it's nature where opposite thing exist together to create meaningful universe

Satan is simply us human beings when we go against God and commit evil. There is no such thing as a 'Satan' being, or a fallen angel named Lucifer, Satan is a metaphorical way of saying that a person is doing evil, so 'they have a Satan inside him/her'. Over time, people actually gave this metaphorical understanding a personality and started to mix it in with what is true, to create falsehood.

Satan never was an actual being, individual, or entity, Satan can be any human being that simply does bad things, they are referred to as A Satan.

An easy way to understand, just replace the word Satan with evil.

Satan by definition is the representation of all that is bad. Saying he can repent is like saying an angel can blaspheme. Which according to some religions it can so I guess in those religions Satan has a chance at redemption but I don't believe that he does cos he's an asshole and god won't have his shit.

Satan is our own inner divinity, only blackened and obscured.

Satan is the person in the grocery store who got to the last bag of onion rings before I could.

*GASP* what if Satan is kind of like the loki of christianity?

Satan is the god of the white crayon.

Because Satan is a christian invention of the clergy it has several definitions. My personal favorite: based on a Celtic goat god. But in the end it's just another imaginary being.