The four Inuwashi Gunjin in the series are the last of their people, a culture of winged warriors who held honor and pride very highly similarly to the Japanese samurai. They address each other as "brother" but it is unknown if they were actually brothers or if it is merely a sign of camaraderie among brothers-in-arms.
Under some circumstances, these four Gunjin were enslaved by Darius Dun, who laid them in shackles and kept them in freezer capsules until they were needed, and used as assassins. Despite this humiliation the Gunjin kept their free will and their sense of honor. As the Turtles in one of their missions protected Cody against the Gunjin even at the risk of their own lives, the Gunjin finally rebelled against Dun and helped the Turtles to bring Cody to safety. When their bands were destroyed, they finally gained their freedom.
The Gunjin, minus their leader, were later kidnapped by an evil alien collector called Aramzedo who planned to add them to his collection of rare lifeforms. With the aid of the turtles, they were freed and Armazedo defeated. They took control of Aramzedo's ship and planned to take all the captured aliens back to their homeworlds. The turtles are worried that the task is too daunting, but the Inuwashi explain that Aramzedo will help them in return for his own freedom.
Behind the Scenes
The kanji that makes up Inuwashi (犬鷲) literally mean "Dog" and "Eagle", respectively. Inuwashi refers to the Golden Eagle. Gunjin's kanji (軍人) comes from "Army" and "Person", taken together, meaning soldier.
According to Playmates Toys' website back when Fast Forward was first introduced, each of the Gunjin had names. These names seemed to be one-to-one counterparts with each of the Ninja Turtles: Leader, Hothead, Sage, and Joker. Though other than Leader it was not revealed which Gunjin had which name. An action figure of the "Leader" Gunjin (simply named by the group's title) was to be released, and was even advertised on the back of the cards of other Fast Forward figures. However, he was never released. Leader was voiced by Brian Maillard while Joker, Hothead, and Sage were voiced by Sean Schemmel and Marc Thompson.