Jinpei, designated G-4 with the codename Jinpei the Swallow, serves as the reconnaissance expert and the youngest member of the Science Ninja Team. His past is shrouded in mystery, as he never knew his parents. He was found as a toddler on Mount Jupiter by Jun, who was fleeing wartime conflict, and brought to the Shirayuki Orphanage where they grew up together. Jun gave him his name and declared the day she found him to be his birthday since his true date of birth is unknown. The series hints that he may be the last surviving descendant of the original Jupiter Ninja Clan, whose leader disappeared around the same time Jinpei was found, though this is never confirmed. He lives and works part-time at the Snack Jun, helping his adoptive sister with cooking, washing dishes, and waiting tables.

Jinpei is a cheerful, excitable, and mischievous child, but he also demonstrates a surprising level of responsibility and maturity for his age. He has a shrewd business sense and handles much of the cooking and nagging of his teammates about unpaid tabs at the snack bar. While often providing comic relief, he is also prone to being a smart-aleck and occasionally uses foul language, behavior that seems to stem from a desire to appear older and less childish. One of his most defining traits is a deep-seated insecurity about not being taken seriously due to his small stature and youth. This often drives him to act recklessly and set off on solo missions to prove his worth and that he can handle just as much responsibility as the rest of the team. He has a fondness for bringing home stray animals, such as seals, whales, and mountain lion kittens, much to Jun's dismay.

On the team, Jinpei's key relationships form his surrogate family. He shares the strongest bond with Jun, whom he usually calls "Oneechan" (big sister), and he clings to her for comfort in times of trouble despite often teasing her relentlessly. He views the team leader, Ken, as a big brother figure and affectionately refers to him as "Aniki," a sign of his deep respect. The large and jovial Ryu is often his partner in crime for comedic moments. Dr. Nambu serves as a father figure and guardian, being responsible for his training and education. Jinpei's need for parental guidance is occasionally highlighted, such as in an episode where a woman mistakes him for her lost son, giving him a temporary experience of having a mother.

Jinpei's character shows noticeable growth throughout the series. His aging is physically apparent even through the progression of the show, partly due to the natural aging of his voice actor. He has formative experiences with young romance, such as when he rescues a rich girl from Galactor kidnappers. His most significant development occurs in a pivotal episode where he journeys to Mount Jupiter to uncover his origins. There, he meets a blind ninja who tells him that knowing one's past is not necessarily a blessing. While he does not learn the truth about his parents, he gains a sense of acceptance and satisfaction from having tried.

As G-4, Jinpei's abilities are built around stealth, agility, and speed to compensate for his smaller frame against adult opponents. He is often underestimated, which he uses to his advantage. His arsenal evolves across different series. In the original, he wields the "American Cracker," a pair of bola-like weapons that can be fitted with blades or explosives. In Gatchaman II, he uses the "Rebound Ball," a super-ball projectile launched from a mitt, and in Gatchaman Fighter, he employs the "Swallow Top," a spinning top attached to a string. In the 1994 OVA, his role is updated to include self-taught hacking skills, using a specialized device to infiltrate enemy computer systems. He pilots the G-4 mecha, a small, versatile all-terrain vehicle (often referred to as the Swallow Helico) that is capable of ground travel, flight, and underwater navigation, ideal for his reconnaissance missions.