Sage was a member of the Inuwashi Gunjin, a race of winged eagle-like aliens who hold a samurai like code of honor. He and his brethren were were enslaved by Darius Dun, who kept them in shackles in freezer capsules until they were needed, and used them 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 Sage and his fellow Gunjin even at the risk of their own lives, sensing the turtles honor Leader and the Gunjin finally rebelled against Dun and helped the Turtles to bring Cody to safety. When their bonds were destroyed, they finally gained their freedom.
He later appears in The Cosmic Completist where he and the other Gunjin (minus their leader) were kidnapped by an evil alien collector called Aramzedo who planned to add them to his collection of rare life-forms. After he and the other Gunjin are freed, they along with the turtles take down Aramzedo. After taking control of Aramzedo's ship Hothead and the other Gunjin planned to take all the captured aliens back to their homeworld. The TMNT are worried that the task is too daunting, but it was explained that Aramzedo would help them in return for his own freedom.