Ken Washio, designated G-1 and known as Ken the Eagle, serves as the leader and primary tactical strategist of the Science Ninja Team. Orphaned at a young age, Ken's father, Kentaro Washio, vanished during his childhood in a presumed plane crash, and his mother later died from illness when he was eleven. Following these losses, Dr. Kozaburo Nambu became his guardian and began overseeing his training in the Science Ninja Technique when Ken turned thirteen. To maintain his civilian cover, Ken works as an aircraft test pilot and an airmail delivery courier, living and operating out of his late father's office at a small, private airfield.

Ken typically maintains a calm, strategic, and approachable demeanor, yet he is not immune to emotional volatility. In high-stakes crises or moments of personal trauma, he can become overwhelmed, often relying on the support of his teammates to restore his focus and resolve. He possesses a strong sense of justice and is compassionate toward the weak, though he can be ruthless when confronting enemies. Ken is also known for certain idiosyncrasies, including a notable lack of financial responsibility, as he frequently accrues debts at the team's meeting place, Snack Jun, and relies on the youngest member, Jinpei, to help cover his expenses. Despite being popular with women and the subject of teammate Jun's romantic affections, Ken is largely oblivious to or avoids romance, seemingly prioritizing his mission above personal relationships.

As the team leader, Ken pilots the G-1 fighter, a vehicle that transforms from a civilian light jet and can dock with the team's mothership, the God Phoenix. His signature weapon is the Birdrang, a rocket-propelled, bladed boomerang that returns to him after being thrown. He also employs feather shuriken for ranged attacks. In later series, his arsenal expands to include the Bird Saucer and the Gatchaman Fencer, a powerful collapsible sword. His martial arts proficiency is extensive, with training in disciplines such as judo, karate, kendo, aikido, and boxing, which he integrates into his combat style. Ken also utilizes Science Ninja Techniques, including the Bird Flash to disorient opponents and the Kage no Bunshin shadow clone technique.

A pivotal relationship in Ken's life is with his second-in-command, Joe Asakawa (G-2, the Condor). Their dynamic is marked by frequent clashes stemming from differing approaches, but these conflicts are underlaid by a deep mutual respect and trust forged through shared upbringing under Dr. Nambu. Ken's primary motivation shifts dramatically upon the revelation that his father did not die but had instead faked his death to infiltrate the enemy organization Galactor as the mysterious pilot Red Impulse. The two are reunited in a critical mission, only for Kentaro to sacrifice himself moments later to destroy a powerful Galactor weapon. This loss plunges Ken into a period of intense grief and depression, which he eventually overcomes, emerging with a hardened resolve against Galactor. This trauma is later exploited by the enemy Gel Sadora, who attempts to use an impersonation of Red Impulse to manipulate him. Throughout the series, Ken's physical condition also becomes a point of development, as he suffers from cellular degeneration caused by the immense strain of using the Hypershoot ability with his weapons and mecha, leading to a steady physical decline in later storylines.