Crona is a meister whose body is uniquely fused with the demon sword Ragnarok. As the biological child of the witch Medusa Gorgon, Crona was subjected to a lifetime of cruel experiments and emotional abuse from a very young age. Medusa replaced all of Crona's natural blood with an artificial substance known as Black Blood, which she also used to melt down and fuse Ragnarok directly into Crona's body. This traumatic upbringing left Crona with an extremely introverted, pessimistic, and submissive personality, characterized by profound shyness and a constant nervousness around others. Crona often expresses an inability to deal with people or situations, frequently uttering the phrase, "I don't know how to deal with..." Due to the abuse suffered at the hands of both Medusa and the bullying Ragnarok, Crona has a fragile and unbalanced state of mind, making them highly susceptible to bouts of insanity and aggression. This instability is amplified by the madness-inducing properties of the Black Blood within them.

Crona's primary motivation throughout much of the story is to gain the acceptance and love of their abusive mother, Medusa, and to avoid being abandoned by her. This desire leads them to commit terrible acts, including hunting and absorbing human souls in an attempt to become a powerful being known as a Kishin. However, Crona's motivations become conflicted after a pivotal encounter with the meister Maka Albarn. Using her soul perception, Maka sees past Crona's violent exterior and into their deeply wounded heart. Instead of continuing to fight, Maka offers Crona friendship, an act of kindness that fundamentally changes them. After this event, Crona is taken into the care of the Death Weapon Meister Academy, where they begin to tentatively experience friendship and a sense of belonging. A strong, protective desire to keep Maka safe becomes a new, powerful motivation, creating an internal war between the longing for Medusa's approval and the unfamiliar feeling of genuine care.

In the narrative, Crona initially serves as a recurring antagonist, acting as Medusa's weapon to further her schemes. Their first battle with Maka and her partner, Soul Eater, results in Soul being badly wounded and infected with Black Blood, which becomes a significant plot point. After being defeated by the DWMA instructor Franken Stein, Crona is brought to the academy, where their role shifts to that of a troubled ally and ward. This period of tentative redemption is cut short when Medusa returns and manipulates Crona into becoming a spy, forcing them to betray their new friends. Crona is ultimately recaptured by Medusa, who removes their memories of the DWMA and brainwashes them, turning Crona back into a weapon of madness. In the final arc, Crona becomes a catastrophic threat, absorbing the Kishin Asura in a desperate attempt to end the cycle of madness, before ultimately choosing to seal both Asura and themself within the moon using their Black Blood to save the world.

The most significant relationship in Crona's life is with Maka Albarn, who is their first and truest friend. Maka's empathy provides Crona with a glimpse of love without conditions, creating a profound emotional bond that persists even when Crona's memories are erased. The relationship with Medusa is a tragic tangle of fear, desperate longing, and ultimately, violent rejection. Despite being her test subject, Crona craves Medusa's love, a need that is never truly fulfilled. The dynamic with their own weapon, Ragnarok, is adversarial; he constantly torments and belittles them, acting as an internal bully, though they are forced to cooperate in battle. While at the DWMA, Crona also forms a gentler bond with the meister Marie Mjolnir, who shows Crona consistent maternal kindness.

Crona undergoes a significant, albeit tragic, character arc, moving from a terrified victim of abuse to a character capable of making their own choices, for better or worse. The initial act of kindness from Maka allows Crona to momentarily resist Ragnarok and Medusa, showing their first signs of agency. However, their subsequent recapture and brainwashing represent a harsh regression. A climactic turning point occurs when Medusa, showing an uncharacteristic moment of affection, fills Crona with a rage that leads them to fatally attack their mother. This act, driven by the cry that Medusa still possessed the "love" she forced Crona to abandon, severs their last anchor, and they fully succumb to their madness. In their final moments, Crona manages to reclaim their sense of self, choosing to sacrifice their freedom to seal away the Kishin Asura, an act of heroic selflessness that redeems their tragic journey.

Crona possesses several notable abilities derived from the Black Blood that flows through their veins. This blood can be hardened at will to create an incredibly durable defense, nullify physical attacks, and instantly seal wounds, granting a high degree of regeneration. It can also be shaped into various weapons, such as Bloody Needles launched from droplets, the Bloody Slicer projectile, or the Bloody Lance. As their power grows, Crona can unleash a devastating technique called Mad Blood, which engulfs opponents in a massive, corrosive wave of insanity-laden blood, capable of destroying entire cities. As a meister, Crona wields Ragnarok, who transforms into a large broadsword. Their signature Soul Resonance technique is the Screech Resonance, where the mouth on Ragnarok's blade emits a deafening shriek that vibrates the sword at high speeds, allowing it to cut through almost anything. This technique has several variations, including Screech Alpha, a shockwave that bites at the enemy. After fusing with the Black Clown, Crona gains the ability to sprout two additional arms from their back, allowing them to wield three demon swords at once, a style called Three-Sword Style. In their most powerful form, Crona can manifest Thorns of Black Blood, which not only serve as offensive and defensive vines but also disrupt the soul wavelengths of anyone they touch, preventing meisters and weapons from resonating.