Jinpei, designated G-4 and known as Jinpei the Swallow, is the youngest member of the Science Ninja Team. His past is shrouded in mystery, as he was found as a toddler on Mount Jupiter by his future teammate, Jun. The circumstances of his birth and biological family are unknown, leading to speculation that he might have been the sole surviving descendant of the Jupiter Ninja Clan after it was taken over by the enemy forces of Galactor. Jun took the child to the Shirayuki Orphanage, gave him the name Jinpei, and declared the day she found him to be his birthday. The two lived together at the orphanage until they were discovered by Dr. Nambu. He was impressed by the boy's agility and nimbleness, which had made Jinpei popular among the other orphans. Dr. Nambu originally intended to recruit only Jinpei for his team, but Jun insisted they would join together or not at all, leading to both of them becoming founding members of the Science Ninja Team.

In his civilian life, Jinpei lives with Jun and works part-time at her establishment, the Snack J, serving as a cook, waiter, and dishwasher. He is approximately ten to eleven years old in the original series, with brown hair and dark eyes. His typical outfit is an orange and yellow t-shirt with the number four on it, paired with blue and white striped bell-bottom pants. He later adopts a cap, crew-neck shirt, and jeans. He is often seen driving his own small dune buggy, for which he carries a special permit, though he prefers to keep a low profile to avoid attention.

Jinpei has a personality that is a mixture of childlike exuberance and surprising maturity. He is generally cheerful, excitable, and mischievous, often serving as the team's idea man and a source of comic relief alongside his teammate Ryu. Despite his youth, he demonstrates a strong sense of responsibility in his work and is dedicated to his mission. He has a shrewd side, as he is the one who keeps track of his leader Ken's unpaid tab at the snack bar. However, being the youngest often leaves him feeling insecure and not taken seriously by the others. This drives a core motivation for him: to prove that his small size does not limit his ability to handle his share of the load. This need to prove himself can sometimes lead him to act recklessly and go off on his own, which frequently lands him in trouble. He has a notable habit of bringing home stray or wild animals, including seals, baby whales, and mountain lion cubs, much to his sister Jun's dismay. He is also known for his occasional foul-mouthed language and terrible puns.

Jinpeis relationships with his teammates are deeply familial. He views Jun as his true older sister, and despite constantly teasing her, he almost always turns to her for comfort in moments of distress, calling her Oneechan. He looks up to Ken as an older brother, addressing him with the respectful term Aniki, and he loyally follows his leadership while also nagging him about his unpaid bills. Ryu is his closest friend among the group, and the two are often partners in comedic mischief. The team's benefactor, Dr. Nambu, acts as a sort of uncle figure for him. The series explores his emotional need for parental support; in one notable episode, he experiences a temporary maternal bond with a village woman who mistakes him for her lost son, a connection he must ultimately sacrifice for the sake of his duty.

As the series progresses and moves into sequel series like Gatchaman II and Gatchaman Fighter, Jinpei shows signs of maturation. His character design is updated to reflect that he is slightly older, and his role evolves with new weapons and technology. In the later OVA, this modernization continues as he is depicted as a computer-educated hacker and gamer, using a sophisticated hacking device in the field. While his core personality remains energetic and loyal, his skillset adapts to the changing contexts of his world, showing a clear trajectory from a young, impulsive child to a more capable and technically skilled operative.

As the team's reconnaissance expert, Jinpei relies on his incredible speed and agility. His small stature is often underestimated by his enemies, which gives him an advantage in stealth and incapacitation-based fighting. Over the course of his missions, he wields an array of specialized weapons. His primary weapon in the original series is the American Clackers, a pair of bolas that could be fitted with blades or explosives for greater effect. In Gatchaman II, his weapon is upgraded to the Rebound Ball, a super-ball projectile launched from a specialized mitt. In Gatchaman Fighter, he uses the Swallow Top, a spinning top attached to a yo-yo-like string. In the OVA, his combat role shifts further towards technology, as he uses a handheld device to hack into enemy computer systems, transmitting data directly to the heads-up display on his helmet. He also pilots unique vehicles, including the Swallow Helico, a small VTOL aircraft for recon, and the Gatcha Four, a tank-like mecha that combines with the others to form the larger Gatchaspartan.