Kunihiko Kimishima is a seventeen-year-old native of the Lost Ground and a close friend and supporter of the mercenary Alter user Kazuma. He has no family of his own and operates as a resourceful businessman who leverages a wide network of contacts to gather valuable information and broker jobs for Alter users. His meeting with Kazuma occurred through these connections and quickly grew into a bond that resembles brotherhood, especially since neither of them has much in the way of traditional family. Kunihiko does not possess any Alter power, but he compensates with sharp wits, a pair of hidden pistols carried under his sleeves, and a willingness to put himself in harm's way for those he cares about. He is characterised by a relaxed and cheerful presence that often supplies comic relief, particularly through his enthusiasm for vehicles. However, his various cars, buggies, and other forms of transportation become a running source of misfortune, as they are inevitably wrecked during Kazuma's escapades or battles.

His personality blends loyalty, courage, and an unshakable optimism. Despite being an ordinary human in a world divided by superhuman conflicts, he never hesitates to defend the Lost Ground from HOLD and HOLY crackdowns or to support Kazuma in dangerous missions. He is protective of Kanami Yuta, the young girl living with Kazuma, and keeps the violent details of their mercenary work discreet for her sake. Kunihiko also harbours an unspoken crush on Ayase Terada, a fellow native, though he mostly keeps this to himself.

Kunihiko's motivations are grounded in friendship and survival. He uses his entrepreneurial skills to provide paid jobs that keep Kazuma and Kanami afloat, but his deeper drive is ensuring the safety of the small makeshift family formed with Kazuma and Kanami. His role in the story becomes dramatically pivotal when HOLY and HOLD attempt to capture Kazuma by using him and Kanami as bait. During this crisis, Kunihiko steals back a buggy, rescues Kanami, and urges her to safety while taking a gunshot wound from pursuing forces. He never reveals his injury and continues to assist Kazuma, bleeding heavily as he drives through the chaos. He only succumbs after the battle, dying from his wounds just as Kazuma returns him to Kanami. This loss sends Kazuma into a blind rage, sparking a violent clash with Ryuho and triggering a second catastrophic upheaval. Later in the series, Kunihiko's spirit briefly returns to encourage Kazuma and help him recover his resolve during a fight against Ayase Terada.

In the manga continuity, his fate diverges: he survives the conflict and eventually marries Ayase, living a peaceful life alongside the people he fought to protect. Across both versions, he stands out as an ordinary individual who repeatedly demonstrates extraordinary loyalty, resourcefulness, and self-sacrifice, cementing his importance not through supernatural means but through unwavering human commitment.