Keita Amishima is a high school freshman who becomes the main human protagonist of this story. Due to his father's frequent job transfers, Keita has spent his childhood moving from place to place, a circumstance that has left him socially withdrawn and disinclined to form close bonds with his peers. At the start of the narrative, he is a self-described recluse at his new school who purposely avoids making new friends, a behavior that leads his classmates to nickname him airhead-kun for his detached and distant demeanor. He often expresses a passive and disinterested attitude, frequently using phrases that indicate he does not care about various matters. Despite this aloof exterior, Keita possesses a kind heart and is capable of taking unexpected and decisive action when he feels a need to help someone else.

Keita's life changes dramatically after he runs away from home to Mount Fuji to meet his only friend from his previous school. There, he becomes caught in a crisis involving a construction vehicle that has been hacked and is running amok. He is rescued by Sosuke Takimoto, an agent of the secret organization Under Anchor, and Takimoto's partner, a small, intelligent robot named Phone Braver 7, who is also called Seven. Keita assists Seven in stopping the rampaging machine, but Takimoto is mortally wounded in the incident. Impressed by Keita's quick thinking and inherent goodness, Takimoto's final request is for Keita to become the new Buddy partner for Phone Braver 7. After this event, Keita is inducted into Under Anchor as an agent-in-training to fill Takimoto's place. For the first eleven episodes of the story, he operates as a trainee until he is officially promoted to the rank of full agent after successfully passing a difficult simulation exercise called the Last Day on Earth.

As an agent, Keita's primary role is to work in the field with Seven to investigate and stop high-tech crimes, often involving hacking, computer viruses, and internet-based criminal conspiracies. His motivations grow from his initial reluctance into a genuine sense of duty and care for his partner. Keita becomes determined to protect Seven and to ensure that his partner does not suffer the same terrible fate as Takimoto. He also becomes deeply involved in the larger conspiracy surrounding the rogue Phone Braver 01, also known as Zero-One, and the enigmatic former agent Magira. This involvement escalates when Keita and Seven are abducted by their enemies. In a critical test orchestrated by Magira, Keita is forced to revisit every major location from his time as Seven's partner while his own phone braver's life hangs in the balance. Even while chained up, Keita demonstrates his courage and compassion by successfully pleading with Zero-One to spare Seven.

The most significant relationship in Keita's life is with his Buddy partner, Seven. Though Seven is a by-the-book and rule-abiding robot, the two form a strong and effective team. Another pivotal relationship forms with Zero-One. After Keita convinces the rogue phone braver to spare Seven, he attempts to take Zero-One under his care, offering him a place at his home. Although Zero-One initially leaves, he eventually accepts the offer and becomes a frequent, albeit mischievous, visitor to Keita's house. Keita also works alongside other Under Anchor agents, such as his lively supervisor Toko Asano, the tech-developer Hirofumi Morishita, and the initially stern agent Daiki Kirihara. Through his experiences with these allies and the dangers he faces, Keita undergoes a significant development, transforming from a lonely, detached teenager who avoids relationships into a committed agent and a dependable friend who will risk his own safety for the sake of others. He is also shown to be a natural problem-solver, able to think on his feet in a crisis, a trait first seen when he helped Seven stop the construction vehicle. While Keita himself does not possess high-tech combat abilities, his notable skill is his capacity for empathy and moral conviction, which directly influences the actions and loyalties of the highly advanced artificial intelligence beings known as Phone Bravers.