Gekka works as a waitress at Kuronekotei café, which employs automata dolls retired from military service. Her lightweight frame often leads others to mistake her for a child. She has light yellow hair and green eyes, dressed in a yellow kimono top patterned with white and red camellias, a white apron, a black skirt, and black boots featuring pink-and-white striped laces. Her backpack houses extendable horizontal wings that require cautious navigation in tight spaces.

Her demeanor reflects her origins as a military reconnaissance doll: blunt, stoic, and formal. She communicates minimally, using only essential words, projecting a cool indifference. She executes instructions precisely without deviation and maintains cautious boundaries, especially avoiding closeness with authority figures.

Deployed as a specialized scout during the war, her lightweight design aided intelligence gathering. This service left deep trauma, including survivor's guilt from instances where delayed intelligence cost fellow automata their lives. These experiences fuel her reserved nature and struggle to adapt to civilian life.

Development emerges through interactions with Haizakura, another automata at the café. Initially dismissive of Gekka's trauma, Haizakura later encourages her to seek purpose beyond military service. This prompts Gekka's tentative steps toward growth, culminating in rare moments like a subtle smile during a nighttime talk. Her journey involves reconciling past guilt with present opportunities, slowly accepting her value extends beyond wartime functions.

Relationships remain largely professional, aligning with her formal approach. She performs duties reliably but avoids deeper connections, except with Haizakura, whose persistent optimism gradually lowers Gekka's emotional barriers, aiding her integration into the café community.

Additional traits include a documented fear of dogs. Her name, translating to "Under the Moon" (月下), reflects narrative themes.