Kazuo Saitō is an 18-year-old high school senior, the youngest member of his team. He stands 181 cm (5'11") tall and weighs 72 kg (159 lbs), with distinctive blonde hair and grey eyes, often seen wearing glasses and a hoodie. His blood type is O. Residing in Tokyo, he helps run a small electronics store owned by his adoptive family; his biological parents died in a car crash during his early childhood. Despite this tragedy, he maintains a predominantly carefree, energetic, and optimistic demeanor.

Saitō works part-time as a convenience store clerk at his school. His hobbies include reading manga, building plastic models like Gundam kits, and playing video games. His favorite foods are hamburgers and spaghetti. While generally lacking physical combat skills and sometimes perceived as a burden during fights, he possesses advanced technical expertise in electronics and machinery. He excels at solving puzzles and memory-based challenges, abilities honed through his upbringing around electronics.

Driven by undisclosed personal circumstances requiring a substantial sum of money, he participates in the illegal Bus Game. His initial naivety about the game's lethal dangers causes friction with teammates Toki Mishiba and Nobuto Nakajyo, who find his upbeat, trusting nature irritating. He demonstrates a tendency to extend goodwill even towards adversaries, reflecting his inherent trustworthiness and people-reading instincts. Over time, his teammates develop a fondness for him despite frequent expressions of annoyance, recognizing his role as an emotional anchor within the group.

His background includes fragmented memories hinting at a twin brother named Shigi. Flashbacks also suggest unresolved trauma connected to his parents' deaths.