Rosemary's past was never revealed. It can be assumed that, like most warriors of the Organization, she was forced into being a half-yoma, half-human hybrid. She was a warrior of the Organization during the era of Hysteria and was a participant in her execution. She was a single digit ranked warrior by the time Teresa entered the ranks, and was ranked No. 1 for a short while before being demoted to No. 2.
Major Story Arcs
A Warrior's Pride
Rosemary, after losing her No. 1 rank to Teresa, vowed to kill her. Using her black card as a pretext for Teresa to meet her, Rosemary lured her to the isolated Godahl Plateau. There, Rosemary fought Teresa, revealing in the process that she awakened prior to sending her black card. Though appearing to have the upper hand at first. Rosemary was easily slain by Teresa once she released some of her yoki. The Organization was unaware of her awakening as Teresa lied about it upon her return.
A Second Final Moment
Rosemary is briefly mentioned by Hysteria, who in her dying moments recalled the Rockwell Hill Massacre. Rosemary was ranked No. 4 during the incident of Rockwell Hill, when Hysteria, that generation's No. 1, refused to be executed despite reaching her limit. Rosemary was the one to land the final blow after Teresa landed a strike which crippled Hysteria.
Rosemary was known to be a quiet and serious individual, who never showed any emotion, and was quite the loner. This changed when Teresa met Rosemary on the Godahl Plateau. During this encounter, Rosemary revealed that she held a deep resentment against Teresa for taking the coveted No .1 rank from her. She consequently hated the Organization as well for taking that rank away from her.
Upon awakening, Rosemary's personality changed dramatically. She was no longer quiet, instead Rosemary was keen on speaking her mind, openly mocking Teresa and the Organization for being stupid. The once reserved and serious individual, became highly emotional and quite arrogant. She reveled in Teresa's pain and gained pleasure at the thought of killing Teresa.