Haruo Niijima, a second-year Kōryō High School student, began compiling detailed student profiles using a palm pilot in the Newspaper Club. His distinct appearance—a long face, sharp nose, pointed ears, and occasionally a forked tongue—earned him the nickname "demon-faced space alien." Antennae sometimes emerge from his head during intense concentration. He typically wears his school uniform but dons a cloak bearing the Shinpaku Alliance insignia when leading.

Initially perceived as a manipulative opportunist, Niijima founded the Shinpaku Alliance aiming for world domination. Yet his motivations revealed deeper altruism. He prioritizes member well-being, often withholding critical information or placing himself in danger to protect them. Examples include provoking Berserker away from injured comrades and forfeiting the D of D Tournament to prevent fatalities at Sho Kano's hands. This protective paternal approach earned genuine respect from the Alliance, including former skeptics like Kenichi Shirahama.

Niijima possesses no formal martial arts training but exhibits exceptional non-combat abilities. His "Niijima Scan" instantly assesses opponents' physical, emotional, and skill-based attributes, detecting hidden emotions like crushes or killing intent. He also senses environmental threats like incoming missiles or concealed enemies. His evasion skills are formidable; he dodged master-level fighters and escaped Berserker through tactical disrobing and agility, employing a signature escape technique called "Shadowless Leap from the Charge" and utilizing urban environments or sewers to avoid capture.

As a tactician, he coordinates combat strategies and intelligence operations, training non-combat members in information gathering and engineering traps at the Shinpaku hideout. His technical skills include building computers from spare parts, creating secure websites, and hacking; he once hijacked a cargo ship while intoxicated. He also saved the Ryozanpaku dojo from financial ruin with a lucrative scheme.

Niijima faced significant adversaries like Ragnarök and Yami. Captured by Loki and Siegfried, he was imprisoned on a departing ship and later orchestrated the Alliance's involvement in rescuing Kenichi's sister. During the Eternal Sunset Arc, his accurate analysis of Yomi leader Kajima Satomi nearly got him killed. He mobilized the Alliance to gather intelligence on Yomi's movements, anticipating attacks and coordinating defenses.

He displays unique vulnerabilities, including an aversion to sacred spaces. Entering a church weakened him severely, requiring others to carry him, while his shadow appeared to struggle independently. Despite his proclaimed "evil" persona, he occasionally assists allies without ulterior motives, such as informing Miu about Kenichi's efforts to protect her play.

His development includes selfless acts like sending Kenichi an uncharacteristically heartfelt message during a Ragnarök ambush to avoid endangering him. By the series' conclusion, he remains the strategic core of the Shinpaku Alliance, balancing world-domination aspirations with unwavering loyalty to its members.