Hideaki Yajima, childhood friend and protector of protagonist Kouichi Hayase, balances loyalty with concealed motivations. His defense of Kouichi from bullies intertwines with a desire to impress Risako Niiyama, their mutual friend whom he secretly admires, fostering tension that deepens when Kouichi gains superhuman abilities as a Machina Factor.

Yajima’s death occurs during a clash with Kouichi amid an assault by the rogue Machina Apparition. The manga leaves his fate ambiguous post-incident, while the anime resurrects him via D-S.O.I.L. nanobots administered by Apparition after it loses its original Factor. This revival grants him a fully restored arm in the anime, contrasting the manga’s unresolved outcome where he retains an artificial limb due to irreversible damage.

Resurrected as Apparition’s new Factor, Yajima pilots its uniquely equipped Machina, featuring optical camouflage, a machine gun, and a high-penetration sniper rifle. Struggling with resentment toward Emi Kizaki’s influence on Kouichi and disillusionment with Machina interference, he engages Kouichi in combat, leveraging enhanced Factor abilities to briefly dominate their confrontation.

The anime expands his arc with the kidnapping of Emi, blaming her for their upheaval, yet he simultaneously supplies Kouichi with crucial tactical intelligence—embodying a conflicted duality of antagonist and ally. Post-resurrection interactions reveal gradual maturity, culminating in reconciliation and collaboration against shared threats. While the anime underscores romantic rivalry with Kouichi over Risako, the manga prioritizes the corrosion of their friendship through Kouichi’s arrogance. Both narratives frame Yajima’s death and return as pivotal to Kouichi’s evolution from hubris to accountability.