Fabia Crozelg is a teenage witch who appears in Magical Girl Lyrical Nanoha ViVid. At thirteen years old, she is a practitioner of the Ancient Belka magic system, but she is explicitly identified as a true witch rather than a conventional mage. This distinction is central to her identity. Her magical style, referred to as True Witch, involves the use of a broom-shaped device named Hell Gazer, the summoning and commanding of demonic familiars known as Petit Devils, and the casting of curses. Her magic color is light purple.

Fabia has blond hair that reaches down to her waist and yellow eyes. She shares a deep connection to the ancient past. Like her acquaintance Einhard Stratos, Fabia carries inherited memories from her ancestor, the original witch Crozelg, who lived during the era of Ancient Belka. This ancestor, nicknamed Cro by the legendary king Claus Ingvalt, bore a grudge after feeling abandoned by the kings she had awaited. Fabia confuses her own identity with these inherited memories, believing the ancient grudge is a mission she must fulfill.

This sense of inherited duty drives her primary antagonistic role in the story. She is first seen participating in the DSAA Intermiddle Championship, where she demonstrates her illusion-based combat skills by defeating an opponent without taking a single hit. Her true goal, however, lies within the Infinity Library. Fabia seeks a document called Eremiahs Memoir to settle what she perceives as her ancestors grievance. To this end, she uses her magic to spy on a group of Ancient Belkan descendants and then intercepts them in the library. She attacks and subdues multiple characters, including Rio Wesley, Miura Rinaldi, and Sieglinde Eremiah, using various spells to seal them inside bottles or transform them. Her plan is ultimately thwarted by the combined efforts of Lutecia Alpine, Vivio Takamachi, and Einhard Stratos. During the confrontation, Vivio reaches out to her, arguing that the current generation is not defined by the actions of their ancestors. This moment shakes Fabias resolve. After Hayate Yagami arrives to mediate, a tearful Fabia apologizes for her actions. She subsequently withdraws from the tournament before her scheduled match with Lutecia.

Following this incident, Fabia undergoes significant personal development. Her misunderstanding is fully resolved when Einhard later tells her that Claus Ingvalt never forgot the witch Crozelg, always considering her a precious friend. This revelation frees Fabia from the weight of the past. She reconciles with the other characters, including Sieglinde and Lutecia, with whom she develops a friendship and addresses by nicknames. By the later chapters of ViVid, she has become a Contract Mage affiliated with the Time-Space Administration Bureau, working to obtain further qualifications. She continues to make minor appearances in ViVid Strike!, where she is seen interacting positively with her new allies and delivering items to the Nakajima Gym, indicating her full integration into the wider circle of characters.

In combat, Fabia relies on her witchcraft. Her familiars are capable of physically restraining opponents, while her own spell list includes Black Curtain for illusion casting, Dark Flame and Comet for attack, and Gravity Bless for area-of-effect attacks. She can also cast space-isolation type force fields and use magic for spying and eavesdropping. Her most powerful technique is Devil Synthesis, which allows her to fuse with her demons, transforming into an adult form with vastly increased power.