Sophie is the protagonist of Sophie's World. She is an inquisitive and spirited fourteen year old who learns just before turning fifteen that her life is the invention of Albert Knag. Sophie learns this and many other things from Alberto Knox, the philosopher who Albert Knag invented as her teacher. Sophie does not just learn from Alberto; she also questions him and shows that she has ideas of her own to implement. By the end of the story Sophie shows that she is a philosopher, because she has the ability to look at things from a different perspective and she can act on what she thinks. Sophie is friendly but not all that social. She is more given to introspection than chatting. In fact, Sophie almost forgets about her one good friend, Joanna, when she starts learning about philosophy. Sophie is critical, and she does not spare those she cares about. Her mother has to listen to much criticism from Sophie throughout the book, and Joanna and Alberto also hear a fair share. Sophie is opinionated and she is interested in saying only what she thinks.
Sophie's teacher, Alberto Knox represents the ideal philosopher. He is never quick to judge and he always thinks about what he is doing. Alberto believes passionately in philosophy, since it helps him understand that his existence is due to the mind of Albert Knag. Alberto is an excellent teacher because he forces Sophie to think things out on her own and does not make things easy for her but he also cares about her and wants her to learn.
Hilde is Albert Knag's daughter and Sophie and Alberto are created for her amusement. Like Sophie, she is a deep thinker, and the philosophy in the book intrigues her deeply. Hilde is also extremely compassionate, and she feels for Sophie and Alberto while her father plays with their lives. She is independent, and proves it by giving her father a taste of his own medicine during his return from Lebanon. Hilde thinks things through but also trusts her instincts over her reason sometimes, and her instinct is what tells her that Sophie actually exists.
Hilde's father, Albert Knag is the brains behind Sophie and Alberto's existence. He creates them as a birthday gift for his daughter, whom he loves deeply. Albert Knag has an ironic sense of humor and cares very much about the world. He works for the UN and he wants people to live in peace and harmony. Albert also very much wants his daughter to see the world (and the universe) as the special place that it is. He wants her to learn about philosophy so that she will be able to think and live in the way that he thinks is proper. Like Alberto, he is a philosopher at heart, and the world itself is enough entertainment for him.
Sophie's mother is one of the funnier characters in the book because she provides a foil for Sophie's philosophical adventures. Mrs. Amundsen thinks that her daughter is losing her mind when she starts spouting off about the differences between humans and animals and how thinking makes one a human being. Sadly, she also represents those who do not think in the world, a population that, Alberto warns Sophie, includes most of the people.
Joanna is Sophie's best friend and she is loyal and friendly, although she does not think about things in the same way that Sophie does. But Joanna also will not turn away from philosophy in the same way that Sophie's mother does. So Joanna shows some promise. Perhaps if she had a philosophy course she would progress in the same way that Sophie did.
Hilde's mother is a very minor character in the book. The relationship between Hilde and her father is much more central, but Hilde mother often provides a stabilizing influence in her daughter's life. We do not know how deep of a thinker she is, but it is clear that she loves her husband and her daughter very much.
Sophie's father is hardly mentioned throughout the book. He sends his daughter a postcard early on and it is clear that he cares for her, but his work keeps him away from home for most of the year.
Hermes is Alberto's dog who works as a messenger, bringing Sophie the lectures on philosophy and later taking her to Alberto. Albert Knag uses Hermes to wish Hilde happy birthday and help ruin the continuity of Sophie's life.
Jeremy is the boy who Joanna begins passionately kissing at the end of the garden party. Sophie invites him because she knows that Joanna wants him at the party.