In its purest definition, an antagonist is a character in a story that stands in the way of our protagonist’s goal. And yes, an antagonist for all intents and purpose is an actual character. In other words, a tornado is not an antagonist and neither is society. Often times we refer to the antagonist as the villain in a story, but we all know that not all stories have a villain and even if they do, they may not be an essential part of the entire story. For example, in TOY STORY, Sid Phillips is our villain, but he is not our antagonist – Buzz Lightyear is the antagonist who stands in Woody’s way of being Andy’s favorite toy.
In elevated stories, as mentioned in the posting WHAT IS A PROTAGONIST? the protagonist is the character he undergoes the change. In these stories, it’s the antagonist (not necessarily evil) who causes this change within the protagonist. In SHAWSHANK REDEMPTION, most would say that Andy Dufresne is the protagonist. But in essence, Red is the one who went through most change as a character and it was Andy who catalyzed that change. So, according to this definition, Andy is the antagonist to our protagonist Red.