In "The Pit and the Pendulum," there really is only one main character, and that is the narrator. The "judges" make brief appearances, mostly as hovering sources of evil that inflict torture on him, but they don't change throughout the course of the tale. The only other character is the french lieutenant at the end that saves the narrator from certain doom at the end. These extra characters are not dynamic; rather, they are static and unchanging, and represent the forces of good and evil in the tale. So, unless you want to consider the rats a character, the narrator is the remaining choice as the dynamic character in this story.
Dynamic characters undergo changes throughout the story, growing and evolving their personalities and responses according to what happens in the story. The narrator, at the beginning of the story, is a very fearful, confused and timid. He submits to the judges, and doesn't fight much in his circumstances. After he is tossed into the dungeon, he is still paralyzed with fear, unsure of what to do, and allows himself to be inactive and give into his terror. However, as time progresses, he gains courage and starts to explore. This shows his personality change, that he is adapting and living up to his circumstances. He very deftly explores in the dark, being very calculated and courageous as he does so. After he falls in the pit and finds himself bound as an eventual victim to the pendulum, he evolves further by not just awaiting death. Instead, he formulates a plan, and enacts it very courageously. His dynamic change of character allows him to escape the ropes.
Throughout the story, the narrator gains strength and courage, wheras at the beginning he was fearful and submissive. This shows the dynamic nature of his character. I hope that helped; good luck!