Diana, the central character of the anime Fencer of Minerva, is a young woman whose life is defined by a dramatic fall from royalty into servitude, a transformation she ultimately embraces on her own terms. Physically, she is depicted as a young woman of about twenty years old, with distinctive light blue hair and red eyes. Her story begins in the kingdom of Doria, where she is the princess, though her position is built on a foundation of tragedy. As a child, she witnessed the brutal coup in which her father, Randis, usurped the throne from King Baasen. In the ensuing attack, Baasen’s son and Diana's childhood friend, Sho, was presumably killed, an event that left her with a deep-seated hatred for the royal status she had once dreamed of possessing.

Years later, Diana's personality is marked by a fierce will and a proactive nature. When her father arranges for her to marry the sadistic Prince Dunan of the neighboring kingdom of Guptia, she refuses to be a pawn in his political games. Demonstrating considerable courage and resourcefulness, she disguises herself as a man and flees the castle, even stopping to rescue a young slave girl named Fina from her abusive traders. However, her bid for freedom fails when she is captured by a tribe of nomads. Despite her protests and claims of being a princess, she is stripped, trained, and brutally whipped to be broken into a slave. It is in this moment of complete vulnerability that Sho, now a wandering swordsman wearing a mask to hide a scar, re-enters her life. He defeats the nomads in combat, and her former friend, not recognizing her at first, claims Diana as his own slave.

This reunion marks the beginning of Diana’s true role in the story and her most significant relationship. She willingly pledges herself to Sho, not just as a slave but also as his lover and most trusted companion. Her motivations shift from escaping a gilded cage to actively supporting Sho in his quest to reclaim his rightful throne from her father. While she endures further trauma, including being recaptured and violated by Prince Dunan, Diana proves to be far from a passive damsel. She possesses notable abilities, including considerable skill with a sword, capable of fighting alongside Sho in battle. Additionally, she is a proficient chess player, a strategic talent she uses to win a slave in a wager.

Throughout the narrative, Diana undergoes a significant development in her understanding of freedom and power. Even after Sho reclaims Doria and her father takes his own life, Diana is offered her liberation. She refuses it. Instead, she chooses to remain by Sho's side as his slave, declaring that she would rather be with him in servitude than be free without him. This choice forges her identity, and she eventually becomes known throughout the land as the Slave Queen. Her character arc is thus a paradox: she rejects the hollow authority of a princess to find a more profound sense of purpose and agency within the framework of a chosen bondage, traveling the land with Sho and inspiring other slaves with her devotion and strength.