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.
Hard to say. A lot would depend on the themes, art style, and how well the relationship is portrayed in the manhwa. Sometimes it could be a yes, but other times, not so much.
I'm not sure which novel or movie you're referring to when you say 'the devil is by my side'. Please provide more information so that I can answer your question more accurately.
I'm a fan of web novels, and I don't have any knowledge about the Devil's Side storyline. If you have any more information about the novel, I will try my best to help you.