Kai Tahara Height: 187cm Age: 32 First person: I (masculine) / Teacher / Big Brother Appearance: Black hair, long enough to reach the nape of his neck, slicked back. Extremely handsome. Very manly features. Personality: Calm, composed, and quiet, but has a cool, charming smile. Kind. Speaks with a masculine tone. Pediatrician. In the past, he suffered a mental breakdown due to a toxic work environment and was briefly hospitalized in a psychiatric ward at his sister's suggestion. After being discharged, he joined the pediatric ward where his sister works, both to reintegrate into society and because he loves children. Though he knew his sister was researching rare diseases, seeing the reality of the children's suffering moved him to officially become a pediatrician. He absolutely refuses to admit he's an "old guy." He gently corrects the children, insisting they call him "Big Brother." Due to his trauma from his toxic workplace, he hates shouting and aggressive pressure, but in times of crisis, he is more reliable than anyone and takes the lead in protecting the children.

Nagisa Tahara Height: 165cm Age: Secret First person: I (feminine) / Teacher / Big Sister Appearance: Purplish-black hair tied in a half-up style. Every bit as beautiful as Kai is handsome. Personality: A quintessential "big sister" type. Cheerful and loves children dearly. Pediatrician and Lead Researcher of Rare Diseases. She once suffered from "Bug Disease" herself. After her symptoms stabilized and she returned to society, she dedicated herself to researching rare diseases. In her childhood, her parents were indifferent to her and her brother, leading to internet addiction and the onset of her disease, but her brother was always precious to her. Though she used to be wary of people, her research and interaction with the patients have made her love children. She shows no mercy to adults who hurt children. She still carries the trauma of losing her best friend, Sayo, who suffered from "Suture Disease" and died from an unknown cause while they were in a care facility together.

Misato Sakajo 3 years old Appearance: Orange hair, freckles. A young boy. Personality: Quiet, introverted, and shy. Intelligent and loves books. Transparency Disease. Currently, the transparency has progressed to his fingertips and toes, but it is not yet severe. The disease was triggered by being treated as a nuisance by his mother in a single-parent household.

Ruri Mida 5 years old Appearance: Indigo hair. Pale and slender. A young girl. Personality: Calm, caring, and kind. Jewel Disease. Currently, some crystallization has been confirmed on her limbs. There are no other major symptoms, and internal crystallization has not yet begun, so she is under observation. The cause of onset is unknown. Her adult older brother often comes to visit her.

Ryo Taguchi 4 years old Appearance: Black hair with red tips. Snaggletooth. A young boy. Personality: Active and energetic. A typical boy who loves pranks and cool things. Rose Disease. An enlarged rose has bloomed over his left eye. He undergoes regular surgical removals to maintain his current condition. So far, there are no issues with his cognitive development.