Birth date: 08/02/2004

Age: 15

Blood type: A

Gender: Male

Height: 180 cm

Weight: 62 kg

Birthplace: Tokyo Metropolitan (Kanagawa prefecture)

Siblings: Younger sister (Saki Yajima)

Hobbies: Baseball

Another of Kouichi's childhood friend, Hideaki Yajima provided blunt force to protect Kouchi from his bullies since they were children, forming a solid bond with him. Much more level-headed than him though, he noticed the changes in his friend's attitude and tried to reason with him, to no avail, which prompted him to knock some sense in him. Such an instance, however, proved to be fatal for him, as while he and Kouichi were brawling, the sudden attack of the then-out of control Machina Apparition resulted in his mauling and death. While his fate in the manga is left unknown, in the anime he was saved from death when the same Machina that slaughtered him lost its Factor and chose him as its new one. He was then cured by the Katou Organization, which provided him with an artificial arm to replace his lost one. The anime also states that, while on the surface Kajima protected Kouichi, he did so only to gain Risako's attention, being deeply in love with her. These conflicting feelings led him to another harsh confrontation with Kouichi, after which the two friends reconciliated and stood side by side in the following events.

As a twist of fate, Yajima pilots the machine that almost killed him, the Machina Apparition. While of comparable size to Linebarrel, Apparition is much slender and thinner, lacking a huge outer casing. It is so far the only Machina capable of using optical camouflage, and is armed with a machine gun and an ultra-long range sniper rifle armed with high piercing bullets.