Lucia Rea Regulus, who is often called Lu by those close to her, is the third princess of the Regulus Empire and becomes one of the nine wives of Touya Mochizuki, earning the position of his fifth fiancee and later the second queen of the Duchy of Brunhild. She is first introduced at twelve years old, four years younger than Touya, and matures to fourteen as the story progresses. Lu has a youthful appearance marked by short silver hair and green eyes, which some sources describe as mint green in color, and she is frequently noted for her kind and straightforward nature.

Her personality is a blend of lively energy and a steady, earnest disposition. She is a sincere and good-natured individual who is deeply devoted to those she loves, often thinking carefully about how she can be of use to Touya. This earnestness comes across as both endearing and admirably loyal. At the same time, Lu possesses a strong competitive streak and a secret inner strength; she hates to lose and can be quite stubborn when her pride is on the line. This determined side occasionally clashes with a shy, self-deprecating tendency, and at times both aspects of her personality emerge together, revealing a character who is earnest yet quietly self-conscious about her own shortcomings.

Lu is motivated primarily by her profound gratitude and love for Touya. Their first meeting occurred during a rebellion within the Regulus Empire, when soldiers were about to kill her. Touya saved her life, and in that moment she fell in love with him at first sight. From then on, her primary drive is to support Touya and prove her worth as his partner, often pushing herself to learn new skills or take on responsibilities that might help him. This motivation leads her to train as a capable fighter and to embrace her role within the growing family of the Duchy of Brunhild.

In the overall story, Lu serves both as a love interest and a valuable combatant. She is a capable warrior who wields dual swords in battle and is also skilled enough to pilot the Chevalier Waltraute, a type of magical mecha known as a Frame Gear. Additionally, she is said to possess an absolute sense of taste, an unusual but notable ability that distinguishes her among Touya's companions. Her place in the narrative expands the scope of Touya's political and personal connections, linking him directly to the powerful Regulus Empire.

Key relationships define much of Lu's character arc. Her bond with Touya is the central pillar of her development; she looks up to him with unwavering loyalty and affection, and her feelings are consistently portrayed as pure and devoted. Among Touya's other fiancees, she grows especially close to Yumina Urnea Belfast, as the two princesses share a similar background and understanding of royal life. This friendship helps Lu integrate into the group of heroines and reinforces her sense of belonging outside her home empire.

Over the course of the series, Lu develops from a sheltered princess rescued in a moment of crisis into a confident, capable young woman who actively contributes to the defense and governance of Brunhild. Her growth is marked by her willingness to step outside her comfort zone, taking up arms and piloting advanced machinery to protect her new family. By the end, she fully embraces her position as Touya's wife and the second queen, embodying a blend of gentle kindness and fierce determination that defines her role within the story.