Haruo Niijima is Kenichi Shirahama and Miu Fūrinji's classmate at Kōryō High School. He started in the Newspaper Club, creating detailed profiles and rankings. Later, he founded and commanded the Shinpaku Alliance, serving as its intelligence officer and strategist. His leadership relies on manipulation and tactical foresight, often withholding critical information to protect members, such as hiding a Yomi member's presence to prevent conflict.

Niijima sports a slim yet solid build, sharp fingernails, gangly limbs, and distinctive facial features often called demonic or alien-like. These include a long face, sharp nose, pointed ears, a forked tongue, and teeth that shift between jagged, fanged, or normal based on his emotions. Intense concentration causes two pointed antennae to protrude from his head. He typically wears his school uniform but dons a cloak as the Shinpaku Alliance leader. Characters frequently refer to him as a "demon-faced space alien."

Initially an ambitious opportunist and comic relief, Niijima evolves while maintaining his proclaimed dream of world domination. He demonstrates genuine altruism and paternal concern for the Shinpaku Alliance, prioritizing their safety above all. This leads him to forfeit the D of D Tournament to protect them from Sho Kano and provoke enemies like Berserker to divert threats. Despite self-identifying as "evil," he consistently aids friends through unconventional methods, earning gradual respect as a leader. Cowardly tendencies manifest as expert evasion in combat, though he risks personal peril for the alliance, such as luring Berserker away from injured members.

Niijima possesses no martial arts training but excels in non-combat skills. His signature ability is the "Niijima Scan," a near-supernatural analytical technique allowing him to assess physical, mental, and emotional attributes within seconds, including hidden emotions, power levels, and killing intent, visualized through a digital HUD formed with his fingers. He exhibits heightened sensory perception, detecting incoming missiles or concealed soldiers, impressing even master-class fighters. As an escape artist, he uses the "Shadowless leap from the charge" to switch positions with adversaries and demonstrates exceptional agility, outrunning skilled fighters like Kenichi. His evasion skills let him dodge master-class sword strikes and survive encounters with opponents like Hermit by escaping through sewers.

He is also a proficient tactician, engineer, and programmer. He built functional computers from spare parts, developed the Shinpaku Alliance's secure website, designed traps against adversaries like the Capoeira team captain, and renovated the alliance's hideout with security systems. His espionage skills include spying on Ryozanpaku masters undetected and stealing keys to Tanimoto's house for unlimited access. Strategic coordination includes delaying Kenichi's fight with Chikage via a birthday party invitation. A peculiar weakness causes him severe physical discomfort in sacred spaces like churches, weakening him and requiring carrying.

Throughout key arcs, Niijima's role expands from instigating Kenichi's early fights to pivotal alliance leader. In the Ragnarök Saga, he spreads rumors of Kenichi's victories, drawing Ragnarök's attention. Captured by Loki and Siegfried, he is rescued by Kenichi, solidifying their bond. He later convinces Kenichi to confront Thor to prevent kidnappings like Honoka's. During the Yami/Yomi Saga, he aids in locating Miu in the Tidat Kingdom and orchestrates intelligence-gathering against Yomi. His leadership is highlighted when he warns the alliance of threats after Jenazard's death and coordinates defenses during Yomi's attacks, including regrouping members during an amusement park assault. His development culminates in mutual respect from alliance members, who acknowledge his indispensable role in their survival and successes.