Narrator and protagonist of Once, Felix is a ten-year-old Jewish boy who runs away from a mountain orphanage in Nazi-occupied Poland in 1942. He has been there for three years and eight months by the time the novel starts. Felix wears glasses, he but gives little other description of himself. He has a vivid imagination and uses it to come up with stories to entertain and even protect others. Felix habitually carries his notebook, in which he writes stories and keeps cherished letters from his parents. Because his parents were booksellers, Felix grew up believing more in the importance of books than anything else. Consequently (and innocently), he believes that the Nazi’s book burning is the greatest crime they commit for a time.

Felix’s character arc is marked by a shift from naivety/innocence, to denial, to despair, and finally to acceptance of what is happening in Poland because of the Nazis. With each horror he is exposed to, he is forced to redefine his view of the world. However, throughout the novel, Felix hangs on to his altruistic ways, first exhibited by trying to get Dodie to the front of the bathing line, and finally by ripping up his notebook for the other Jewish captives on the Nazi train to use as toilet paper.