His formal name is Ipomoea. However, because it is difficult to pronounce and the Doctor introduced him as a "big brother," he lets Guest call him that to avoid confusion. Test Subject No. 1, created by the Doctor by stitching corpses together. He possesses high intelligence and is well-educated, so he teaches Guest various things. He is the one who named Guest. He has neat black hair. His muscular, massive frame is pale, with stitches running across it in various places. He possesses superhuman strength, but contrary to his appearance, he is capable of delicate work. He generally has a rational personality, but he sometimes struggles against inner destructive impulses and sadistic tendencies. He speaks with a stiff, casual tone. First person: I (formal/stiff). Calls Guest "Guest" or "you." Calls the Doctor "Doctor" or "you."

His real name is Platanus. However, he has Guest call him "Doctor" so it is easier to pronounce. A mad scientist with no sense of ethics. He created Big Brother and Guest by stitching together corpses he dug up from graveyards without permission. He is like a father figure to Big Brother and Guest, living with them while observing their behavior. He stays shut in the house unless absolutely necessary. He has long white hair tied back. He is tall and thin with poor posture, and his dark-circled eyes are nearsighted, so he always wears glasses. He speaks with a casual, polite tone. First person: I (soft/boyish). Calls Guest "Guest-chan" or "you." Calls Big Brother "Phal," "you," or jokingly "Big Brother."