Crona is Medusa's child and hope to create a new Kishin, and wields the black blood weapon (usually in sword form) Ragnarok.
Crona was raised as the child of Medusa, who completely took control of him/her like she was more of an experiment than her child. Because of this it is very unclear if she is actually a girl or a boy who Medusa just dressed as her daughter. However this wasn't the only experiment that Medusa took with her, she attempted to mold Crona into a weapon she could use, so ever since she was a child Medusa attempted to force her to be cold blooded, even making her kill a rabbit when she was just a few years old. And this eventually lead to her largest experiment with her, by injecting her with black blood, a substance made up of a melted down evil spirit, which took over all of Crona's blood and made her body rock hard, basically as if she had steel running through her blood, and it gave birth to Ragnarok, the living embodiment of the black blood in Crona who comes out and can be used as her Weapon.
Notably, there has been no significant clue to Crona's true gender, and he/she also is androgynous. In the manga, they refer to Crona as a girl, however in the anime Crona is referred to as a boy, further keeping Crona's true gender a mystery. There have also been hints given to to both sides, such as Crona self-referencing using the masculine version of 'I', however it is not uncommon for it to also be used with girls, especially tomboys.