Red Sonja, the She-Devil with a Sword, is a fictional character, a low fantasy sword and sorcery heroine created by Roy Thomas and Barry Windsor-Smith. She first appeared in Conan the Barbarian #23 (Marvel Comics). The character was loosely based on Red Sonya of Rogatino in Robert E. Howard's short story "The Shadow of the Vulture" (The Magic Carpet, January 1934), which Roy Thomas rewrote as a Conan story for the Marvel comic. Thomas also somewhat based Red Sonja on another character - Dark Agnes de Chastillon (also known as Agnes de Chastillon, Dark Agnes, Agnes de La Fere, and The Sword Woman), who is a fictional character created by Robert E. Howard set in 16th Century France. The stories were not printed until a long time after the author's death.
The character of Agnes was beaten by her father and almost forced into an arranged marriage. She avoids this by killing the bridegroom and running away. She meets Etienne Villiers, who at first attempts to sell her to a brothel, and Guiscard de Clisson, a mercenary captain who trains her as a swordswoman. When de Clisson is killed, Agnes heads for Italy with Villiers.
Like the later character Red Sonja, who was based on another creation of Robert E. Howard, Agnes has red hair and a short temper. Unlike that character, her skill is a mixture of innate talent and training, compared to Sonja's skill being a divine gift.