The devil in manhwa can take many forms. It could be a traditional evil character with horns and a pitchfork, or it could be a more symbolic representation of evil or temptation. It all comes down to the imagination of the author and the message they want to convey through the story.
To survive, you should build alliances with the right characters. Also, stay away from obvious traps and don't trust everyone easily. Keep a low profile when necessary.
It's not based on a true story. The plot and characters of 'The Devil You Know' are the result of the author's imagination and creative writing. There are no real-life counterparts for the events and individuals depicted in it.