Well, the librarian is the central figure. However, we can expect other characters as well. There could be a young prisoner who finds solace in the books provided by the librarian. There might also be an older, wiser inmate who gives advice to the librarian on how to keep the library going. And perhaps there are some guards whose actions or inactions have an impact on the librarian's work and the existence of the library within Auschwitz.
The librarian is surely one of the main characters. There might be some fellow prisoners who help the librarian in the tasks related to the library, like collecting books or protecting them. Also, there could be characters that represent the different types of people in the camp, such as the ones who have lost all hope and those who are inspired by the librarian's efforts, all of which are important to the story.
The main character is likely the librarian. But there may also be other inmates or even some of the guards that play important roles in the story as they interact with the librarian and the library in the concentration camp.