Kyouko Machi is a dullahan, a rare demi-human whose head remains completely detached from her body. A non-flammable green flame connects them at the neck, intensifying with strong emotions like embarrassment. She constantly carries her head in her arms, limiting her use of both hands and requiring lifestyle adaptations such as school permission for a backpack. Practicality dictates her short, spiky ginger hair; longer hair would make her head slippery. Her tall body is physically fit with muscular arms from carrying her head and noticeably voluptuous, sometimes causing issues like sweat showing in warm weather, leading her to wear extra layers like a blue vest.

Shy, patient, and highly intelligent, she consistently achieves top academic rankings. She initially struggles socially due to others' discomfort with her detached head, avoiding inviting friends home during middle school. Gradually, she becomes more expressive through body language and uses self-deprecating "dullahan jokes" to ease tensions, advised by her biology teacher Tetsuo Takahashi. She deeply craves physical affection, finding comfort in human contact like having her head hugged or her body held, and dreads isolation. Resilience surfaces when coping with setbacks, including romantic rejection.

Born normal, she transformed into a dullahan suddenly as a toddler when her head detached. Partial Irish ancestry from her mother influences her hair color and potentially her condition. A childhood incident demonstrated extreme separation tolerance: while saying goodbye to grandparents, her head stayed with her parents as her body accidentally boarded a bullet train from Tokyo to Okayama without harm. Medical exams confirmed a neck structure existing in "another space," suggesting interdimensional connections or wormhole-like properties.

Her abilities include maintaining sensory and functional links between head and body across vast distances. Vision, hearing, smell, and taste remain localized in the head, while touch and proprioception extend to the body. She manipulates objects skillfully without visual guidance, such as writing accurately while looking away. The green flame emits no heat but produces pressure when touched. Her unique physiology allows unconventional actions like specialized tongue tricks.

Relationships define significant development. She trusts Takahashi deeply through his supportive mentorship, a trust evolving into a romantic crush. She seeks comfort in his embraces, though physical proximity sometimes causes mutual awkwardness. Encouraged by friends, she arranges an experimental "head-only date" with him and later confesses her feelings, but he gently declines due to their student-teacher relationship and his growing affection for colleague Sakie Sato. Machi recovers through camaraderie with fellow demi-humans Hikari Takanashi and Yuki Kusakabe. Her friendship with Hikari includes playful teasing and support, though she dislikes Hikari's mischievous experiments with her body. She bonds with Yuki after overcoming initial misunderstandings about Yuki's aloofness. She shares romantic advice with Sakie Sato, as both harbor feelings for Takahashi.

Her character arc progresses from isolation to integration. Early episodes highlight daily challenges, like a fever separating her head and body in different locations, requiring school accommodations like backpack usage. Takahashi's encouragement helps her embrace humor about her condition, improving peer interactions. Academic excellence and interactions with university researcher Souma inspire her to pursue dullahan research, aiming to unlock biological secrets through firsthand study. After her confession to Takahashi fails, she focuses on self-improvement and research goals, demonstrating emotional resilience.

Trivia includes her nickname "Macchi" from Hikari, the "watermelon" metaphor referencing her physique, and promotional materials often depicting her head near her neck to avoid startling audiences. Medical scans of her neck connection spurred theories about multidimensional biology, aligning with Souma's quantum mechanics research. Her height and physical proportions create a noticeable contrast with her timid personality.