Crona is a meister whose weapon, a demon sword named Ragnarok, resides permanently within their own blood. The child of the witch Medusa Gorgon, Crona was created and raised to be a tool in her complex schemes. From a very young age, Crona was subjected to horrific experimentation, the most significant of which was the complete replacement of their natural blood with a malevolent substance known as Black Blood, into which the demon sword Ragnarok had been melted. This process forged an inseparable, and deeply dysfunctional, symbiotic bond between Crona and Ragnarok. Medusa further shaped Crona’s early years through brutal training that included forcing the child to kill small animals, all in an attempt to cultivate madness and suppress any sense of normal human empathy.

Crona’s personality is defined by profound timidity, submission, and a pervasive sense of self-loathing, all of which are direct results of Medusa's abusive upbringing. Naive and socially inept, Crona often responds to stressful or unfamiliar situations by retreating to a corner of a room, referring to it as "Mr. Corner," and frequently expresses an inability to cope by saying, "But I do not know how to deal with it." Ragnarok, who manifests as a bullying, childlike entity, constantly exacerbates this insecurity by giving Crona noogies, stealing food, and issuing threats. This dynamic leaves Crona dependent and largely unable to act without Ragnarok’s often-cruel prompting. The character is presented with a deliberately ambiguous gender, a choice made to underscore their fractured identity and lack of selfhood, leaving the character's gender unspecified throughout the series.

Initially serving as an antagonist, Crona is sent by Medusa to collect souls, both good and evil, to further her goals. However, the trajectory changes upon a fateful encounter with the meister Maka Albarn. During their battles, Maka perceives the immense sadness and emptiness within Crona's soul. Rather than continuing to fight, she offers Crona friendship and physical affection, something Crona had apparently never experienced. This act of genuine kindness becomes the catalyst for Crona’s gradual shift away from Medusa’s control. Enrolling at the Death Weapon Meister Academy, Crona works alongside Maka and her friends, though this transition is fraught with difficulty due to past trauma and Medusa’s lingering influence, which often forces Crona back into a role of reluctant betrayal.

Key relationships define Crona’s journey. The connection with Maka Albarn is the most crucial, as her compassion and ability to tune into Crona’s soul wavelength provide the first real glimmer of hope and a path toward redemption. The relationship with the weapon Ragnarok is one of codependency and abuse, though despite his constant torment, Ragnarok’s survival is tied to Crona’s, leading him to provide healing and protection in battle. In stark contrast, the bond with mother Medusa is purely manipulative and cruel, with Medusa viewing Crona as a failed experiment or a disposable servant, demonstrating no genuine maternal affection. As the story progresses, Crona also develops tentative connections with other main characters like the ninja Black Star, often reacting to his loud personality with bewilderment.

Throughout the series, Crona undergoes significant development, transforming from a withdrawn and controlled weapon into a person capable of forming bonds and making choices based on loyalty rather than fear. This internal growth is visually represented by Ragnarok shrinking in size as Crona’s dependence on him for emotional support lessens. Even when manipulated back into serving evil, Crona’s fundamental desire for friendship and a place to belong remains a powerful driving force, leading to moments of profound sacrifice in the final struggle against the Kishin.

Notable abilities stem almost entirely from the Black Blood and Crona’s fusion with Ragnarok. The Black Blood provides immense defensive capabilities, making Crona highly resistant to cutting and piercing attacks, and offers a degree of self-healing. In combat, Crona wields Ragnarok, who can transform into a large, jagged sword. Their attacks are sound-based, with powerful slashes named Screech Beta, Alpha, and Gamma. Crona can also perform a technique called Blade Slight, which involves cutting their own wrist to fling projectile blades of hardened blood. Another ability, Bloody Needles, turns any spilled drops of blood into hundreds of piercing projectiles. As Crona consumes more souls or absorbs powerful entities, such as the Black Clown, their power increases dramatically, allowing them to create enormous constructs of Black Blood and generate madness wavelengths capable of affecting entire cities.