I was thinking more about antagonists today (and protags and villains) and realized that The Breakfast Club is a great tool to discuss this topic. And actually, The Breakfast Club is a great tool to discuss multiple topics, so if you haven't seen it (or haven't seen it in a while), rent it and watch it. That way I can refer to it and we'll all be on the same page.
I grew up on John Hughes' films, so they hold a special place in my heart. To me, The Breakfast Club is a screenwriter's movie. It epitomizes the idea of "simple plot, complex characters." The plot: 5 kids spend a day together in Saturday detention. What makes it so memorable? The great characters. And the fantastic dialogue. If you want to study some good dialogue, read this script.
Let's start with the villain, because he's easy. It's the Principal. He's the bad dude. He's the judge, jury, and jail keeper. He's got the power and could do serious harm. And, even though these kids don't get along with each other, he's the only one they would unite against.
This would most likely be billed as an ensemble cast, and each of the five students does have a character arc. Antagonists and allies can have character arcs. Heck, even villains can have character arcs. However, only one of the students is the true Antagonist.
Which student is the only character who is indispensable to this story? Meaning, if this character were removed, the detention would have been business as usual, students doing their work, staying out of trouble, etc.
It's John Bender (Judd Nelson). He's the one who teases, goads, pushes, and tests everyone else. He's the antagonist. It's because of his actions the rest of the students are brought into this story.
And if you ask me, Claire (Molly Ringwald) is the true Protagonist. I say this because I believe she had the biggest character arc and she was the most intimately affected by the antagonist. She has a more personal relationship with him and she takes the largest risk. In this way, she becomes more indispensable than any of the remaining students.