Croix Meridies arrives at Luna Nova Magical Academy as the new professor of Modern Magic, presenting herself as a visionary who can solve the institution's energy crisis with her pioneering field of Magitronics. Her introduction is carefully orchestrated, as she secretly engineers the fairy strike that she later appears to resolve, installing her Sorcery Solution System under the guise of helping the school.

Behind her charming and composed exterior lies a meticulous and calculating individual. Croix is a prodigy who was known for her academic excellence during her own student days at Luna Nova, having been named a Moonlit Witch and winning the Luna Nova Cup broom relay. Despite her accomplishments, she harbored a deep-seated ambition to become the greatest witch by unlocking the Grand Triskelion and obtaining the world reconstruction magic. This dream was shattered when the magical staff Claiomh Solais chose her close friend Chariot du Nord instead of her, an event that planted the seeds of bitterness that would define her adult life.

Croix's personality is defined by a cold, logical approach to achieving her goals. She plans her actions with precision, having secretly observed the young witch Akko Kagari for months before making her presence known at Luna Nova. Her interest in Akko is purely transactional, as the girl is the new wielder of the Shiny Rod and Croix seeks to extract data from the staff to create her own imitation, the Noir Rod. She manipulates events and people without concern for collateral damage, demonstrated by her willingness to endanger Akko's life only to conveniently rescue her, thereby earning the girl's trust.

However, Croix is not presented as irredeemably evil. Her primary motivation stems from a genuine concern that magic is being forgotten by the modern world, and she believes that her methods are necessary to prevent this loss. This positions her as a foil to Chariot, who believes magic should be used to make people happy. Croix dismisses this idealistic view as naive and directly responsible for Chariot's failure. Her ruthless pragmatism has limits; when her actions cause Chariot to fall unconscious from her broom, Croix panics and cries out for her to wake up, revealing she never intended to cause such extreme harm.

The heart of Croix's character lies in her complex relationship with Chariot du Nord, who now teaches at Luna Nova under the name Ursula Callistis. The two were once close friends, with Croix initially supporting Chariot despite her resentment over not being chosen by the Shiny Rod. Their falling out occurred when Croix offered Chariot an untested spell called Dream Fuel Spirit to enhance her performances, deliberately withholding that it drained the magical potential from audience members, including a young Akko and Diana Cavendish. When Chariot discovered the truth and ended her career as Shiny Chariot, Croix coldly reaffirmed that Chariot never deserved to be chosen, cementing their estrangement.

Throughout the series, Croix actively works to undermine Ursula, believing that destroying her former friend's last hope of fulfilling her dream is a worthy goal. She reveals the painful truth about Chariot's past magic-draining performances to Akko, not out of honesty but as a calculated move to break the bond between student and teacher. Despite this antagonism, Ursula never abandons her former friend, and when the Noir Rod turns on Croix, Ursula risks her own life to save her. This act of forgiveness prompts a genuine change in Croix, who finally reconciles with her old friend and vows to find a cure for the curse afflicting Ursula's ability to fly.

As a practitioner of Magitronics, Croix possesses notable abilities that set her apart from traditional witches. She has created numerous technological devices that can absorb and convert magical energy from the Sorcerer's Stone, manipulate emotions to generate fuel for magic, and even read memories. Her Pixels are small, cube-like machines capable of possessing objects and ghosts, while her Noir Rod serves as a corrupted alternative to the Shiny Rod, designed to force open the Grand Triskelion rather than earn the right to unlock it. Croix is also highly skilled in traditional magic, able to match Chariot in combat and perform memory manipulation on a large scale. Her development from a bitter antagonist who believes feelings cannot change the world to a woman who accepts that magic should bring happiness represents her full character arc, as she ultimately chooses to walk a different path than the one she spent years preparing.