Being a plot device is hard work. When you're playing a character, you at least get to make decisions that can affect the other characters and the course of the plot, but as a plot device, your role is pretedermined and single-fold. You are written into the story as nothing more than an afterthought explaination that makes some part of the story possible. Even stock archetype roles have more literary merit and personal freedom than the character used as a plot device. As an archetype, you are given a basic model of your personality and characteristics, but the role itself serves a particular purpose based on choices made and the course of the story. The plot device is different because it is not necessarily a character, but when it is, and when you can spot it, it means that the entire history leading up to the action of the story serves naught but to fulfill a singular purpose. Who he is or anything about him doesn't matter; what matters is his existence alone, unlike the character based on an archetype, who can be a fully dynamic character with motivation and everything.
The character I'm refering to today happens to represent the temptation of the protagonist. This is a basic model to set up the conflict of a story and is also a vignette in many journey stories, and in this particular story it happens to come in the form of a minor character. Unlike the antagonist, who perform many of the same actions, he is not given a motivation or full dynamic characterization. He is little more than a test the protagonist must pass. It is clear that his role is not merely characature because the same literary end could be attained by the use of any of the classic symbols - apple, serpent, etc, or any scenario the author cared to write in. As I said, his identity and his story are irrelevant. He could be anybody doing anything; he just happened to be the one who came across the protagonist at the right time to fill this role. Because the story revolves around the protagonist, he is not a real person, and all the influences and nuances that molded every cell of this intety, as well as his future, wherein the chance meeting with this story's protagonist will become nothing more than a footnoted plot device in itself, aren't even discussed. He exists because he has to exist in order for the protagonist's business to go as it has to. The antagonist, on the other hand, has specific motivations and reasons for his actions that help to shape the story as much as those of the protagonist, and he is very much a main character of the story because of this and not in spite of it.
Our plot device friend somehow seems to usurp a lot of screen time and have a major inpact on the psyches of the characters in spite of his functionally minimal role. I suppose that just means he's damn good at his job, though, as the purpose of his role seems to be to impair judgement and hinder the protagonists progres in a psychological way. He's sure trying damn hard to write himself into a better role, even a lead, but then again, maybe that's the very temptation he's written into this story to provide. And he's come pretty close to leading the whole cast into drowning in the well of his own pen. I suppose the final direction of this story lies in the hands of the author. The only question is, "Who's doing the writing?"