Crona - Yenra Picture Wiki
Crona is androgynous in appearance, leaving the character's gender unknown, though Crona is addressed with male pronouns in the Yen Press and Funimation translations of the manga and anime, respectively, for convenience.
As a child, Crona's blood is replaced with a sanity-detering substance called "Black Blood" which Ragnarok's essence is mixed into. This forms a symbiotic relationship between Crona and Ragnarok, the latter of whom appears from Crona's body in a black, vaguely humanoid form when not in his weapon form. Because of Medusa's abuse and Ragnarok's bullying nature, Crona is extremely reclusive and fearful of almost everything. Unlike normal weapon and meister pairs, who synchronize their soul wavelengths to increase their strength, Crona uses a skill called Scream Resonance together with Ragnarok, who emits a scream from the mouth on his weapon form's blade that causes it to vibrate and deal additional damage. The black blood can also harden to prevent physical damage, and can extend from Crona's wounds to be used as an additional weapon.