With the exception of Josh and Sophie Newman all of the main human characters in the books are real. As for the mythological characters, Michael Scott has gone back to the original myths and legends.
Read more about the characters here.
The Catacombs of Paris
In the Magician, the underground warren of tunnels beneath Paris play a big part in the story. Many people believe I’d made them up, but they really do exist and all the details in the book are true to the facts. Here is a fascinating video, courtesy of NPR and National Geographic.