Mio Ibuki attends Tokyo Metropolitan Advanced Nurturing High School, starting in Class 1-C before moving to Class 2-B. She stands 159cm tall with short blue hair featuring one distinctive upright strand and purple eyes. Her standard wear includes the school uniform with a blue bowtie over an orange shirt, switching to a red tracksuit during special exams like the island survival test. Preferring solitude over socializing, she remains isolated within her class. She visibly disapproves of cruelty, flinching at Ryūen Kakeru's violent acts yet following his leadership due to recognizing his strategic prowess. She rejects pity or condescension outright, refusing help even when injured. Experiences with deception under Ryūen make her anticipate mistrust, responding defensively to inquiries—though unexpected trust, like Kiyotaka Ayanokōji believing her innocence during an underwear theft, surprises her into nervous blushing. She exhibits curiosity about interpersonal dynamics, particularly Kiyotaka's relationships, and demonstrates deceptive actions herself by infiltrating Class D during the island exam to steal Suzune Horikita's keycard without remorse. During the Cruise Ship Special Test, she confronts Ryūen about his methods only to be dismissed. Albert Yamada intervenes as she persists, leading to a physical altercation where she rebuffs Kiyotaka's attempted help and storms off. Injured during the island survival test, Kiyotaka's group brings her to Class D's camp despite inter-class rivalry objections. After the theft incident, she confronts Kiyotaka about suspicions against her; his expression of trust catches her off guard. Later, she exploits chaos from a campfire to steal Horikita's leader card while Horikita bathes, capitalizing on Horikita's fever to defeat her in combat. Kiyotaka deduces her role as a Class C spy and manipulates events to let her succeed in identifying Class D's "leader," only to change leaders later and outmaneuver her class. Following Class C's loss, she apologizes to Ryūen, who forgives her. In later arcs, she participates in Ryūen's rooftop interrogation of Kei Karuizawa but is defeated by Kiyotaka during his intervention. She attempts to stop Ryūen from quitting school and aids him during a class expulsion vote by redirecting scrutiny toward Shiho Manabe. Outside exams, she visits a fortune-teller in Keyaki Mall during summer vacation but requires pairing up; encountering Kiyotaka, they enter together. She later meets him at a winter vacation movie screening, displaying visible discomfort at the coincidence. She openly dislikes female classmates, describing herself as "prickly" and admitting social interactions induce anxiety. In Year 2, she initially refuses group formation during another island exam but later assists Horikita against Ichika Amasawa. On a cruise ship, she negotiates an information-sharing pact with Horikita and plans a rematch. During the sports festival, she pressures Kikyō Kushida to return after a breakdown. At the cultural festival, she wears a costume after losing a poker bet with Ryūen and collaborates with Horikita to investigate mysterious threats. Her physical aptitude is graded B- (64/100), specializing in capoeira-derived martial arts. She lands swift strikes on opponents like Horikita but is consistently overpowered by top fighters like Kiyotaka, Albert, or Ryūen due to inferior reflexes or strength. Academically, she scores averagely (C grade, 53/100) with weaker analytical thinking (D grade, 30/100). Relationships remain complex: she respects Ryūen's leadership while despising his ruthlessness; views Ishizaki Daichi as a comrade but clashes with Shiho Manabe; develops a rivalry-turned-alliance with Horikita; and harbors resentment toward Kiyotaka for outwitting her while acknowledging his capabilities. She initially admires Kushida's apparent kindness but later appreciates her honesty after seeing her true nature.