Hanaka Busujima is an Alchemist Warrior affiliated with the Alchemist Army, introduced as a member of a specialized team within that organization. She is sixteen years old and a student at Ginsei Private Academy, balancing her life as a high schooler with her duties fighting supernatural threats. A defining and immediately noticeable feature of her appearance is that she is always seen wearing a gas mask, a visual trait she maintains from her first appearance until the epilogue chapter of the story.

Her primary weapon is the Buso Renkin known as Aerial Operator, which corresponds to Kakugane number thirty-nine. This device manifests as the gas mask and its accompanying apparatus, granting her a unique set of abilities centered on the manipulation of gases. Aerial Operator allows her to mix various gases to create different effects, including producing poisonous gas, knockout gas, and even laughing gas. The mask also serves a protective function, filtering the gases she generates to keep her safe from their effects. The goggles of the mask provide enhanced vision, allowing her to see over long distances like binoculars, to magnify objects like a microscope, and to see in the dark with night vision. Her most subtle but notable innate talent is her ability to accurately gauge the oxygen density of the air around her.

Despite her combat-oriented role, Hanaka’s personality is marked by deep-seated shyness. This trait is so pronounced that she keeps her Buso Renkin equipped at all times because she is extremely shy about showing her face to others. Her true face is only revealed in the epilogue chapter of the manga, highlighting the depth of her introversion. Consistent with her abilities and personality, she has a strong preference for clean air and quiet, uncrowded places, while disliking polluted air and large crowds. Her hobbies include walking in the woods and practicing aromatherapy. She is also defined by her caring and loyal nature, particularly in her relationship with her teammate Sekima Hiwatari, for whom she cares deeply and constantly worries about.

In the narrative, Hanaka serves as a supporting member of her team. Her role is not primarily focused on direct, head-on combat but rather on providing tactical support and area control. Her team affiliation varies between different versions of the story; in the anime adaptation, she is a member of the Victor Hunt Squad, a group assembled to find and eliminate the powerful being known as Victor. Her teammates on this squad include Inukai, Maruyama, Chitose Tateyama, and Shusui Hayasaka. During their mission to confront Victor, the team was easily overpowered and defeated. In this critical moment, Hanaka utilized her abilities to release a large cloud of smoke, creating a diversion that gave her teammate Chitose enough time to teleport the entire group to safety. In the manga, she is a member of the similarly purposed Re-Extermination Squad. Her strategic value is further demonstrated in a notable combination attack where she spread oxygen over several kilometers to strengthen Hiwatari's powerful fire-based technique, Blaze of Glory, enabling him to launch an attack of immense scale. Her character undergoes a small but meaningful revelation by the story's end, finally removing her mask and embracing her own identity more openly.