Haruki Yamauchi, a student in Tokyo Metropolitan Advanced Nurturing High School’s Class D, forms the "3 Idiots of D-Class" alongside Ken Sudō and Kanji Ike, notorious for their academic struggles and rowdy conduct. Prone to compulsive lies, he inflates his accomplishments, boasting fictitious titles like national table tennis champion or baseball star. Despite rock-bottom academic and athletic rankings, his admission—secured through a disputed recommendation—fueled speculation about unethical maneuvering. His lecherous demeanor targets female peers such as Kikyō Kushida and Airi Sakura, fixating on their looks. During an island survival exam, he abandoned pursuit of Kushida as hopeless, shifting attention to Sakura with misplaced faith in niceties winning her heart. A spin-off volume reveals his eventual confession to her, rebuffed, followed by a botched plot with Ike to infiltrate the girls’ locker room, cementing his stagnation. Clashes with authority and peers define his conflicts. After carelessly colliding with Arisu Sakayanagi at a training camp and offering a half-hearted apology, he earned her lasting enmity. Sakayanagi later exploited his gullibility, feigning romantic interest to recruit him in a scheme expelling Kiyotaka Ayanokōji by framing Manabu Horikita. The plan unraveled when Sakayanagi redirected decisive votes against Haruki, culminating in his expulsion with 33 condemnatory votes. Public condemnation by Horikita and violent outbursts laid bare his self-sabotaging nature. Post-expulsion, he enrolled elsewhere without legal fallout, morphing into a recurring subject of fan debates and memes. His arc underscores perpetual failure, social isolation, and fallout from manipulation. Bonds with Sudō and Ike frayed irreparably, the pair ultimately branding him a "terrible monster." Fleeting camaraderie, like aiding injured peer Mio Ibuki on the island, failed to offset his reputation as a perpetual agitator. Standing 168 cm with brown hair and crimson eyes, he typically dons the school uniform. His expulsion, marked by its visceral intensity, sealed his trajectory as a cautionary emblem of accountability and fractured social dynamics within the series.