A Klingon agent surgically altered to look human. His career was undone by James T. Kirk and a tribble, making him an outcast from his people for the rest of his life. The spy, whose Klingon name is unknown, posed as Arne Darvin, a Federation official assisting Nilz Baris in the development project for Sherman's Planet in 2267. His mission was to derail Federation colonization efforts by poisoning a shipment of quadrotriticale grain being stored aboard Deep Space Station K-7. Darvin was exposed by Kirk when the captain stuck a tribble in his face and it squealed violently — a reaction the furry creatures have only toward Klingons. Darvin was arrested, and in the ensuing diplomatic embarrassment Klingon Intelligence turned its back on him, leaving Darvin with his human appearance and forcing him into exile. He spent the next 105 years eking out a meager living posing as a human merchant named Barry Waddle, blaming Kirk for his fate. Then, in a final indignity, he was trapped in Cardassian space by the Klingon invasion of 2372.
The next year, Darvin learned that the Cardassian government was returning the Orb of Time to Bajor via the Federation starship Defiant, so he seized the opportunity to exact revenge upon Kirk and change his destiny. After coming aboard the Defiant as a passenger, he broke into the cabin holding the Orb and stunned the Bajoran deputy guarding it, then used the Orb to send the Defiant back in time to the year 2267. He immediately beamed over to Station K-7, wiping the transporter log to cover his tracks, and began his plot to change time. He planted a bomb in one of the tribbles that had invaded the quadrotriticale storage bin, and set it to go off when a pile of those tribbles would fall onto Kirk's head. But the Defiant crew eventually caught up with him, and he was captured and brought back to the ship. He insisted to Odo and Worf that he was one of the greatest heroes of the Klingon Empire, and dreamed that he would someday have a statue in the Hall of Warriors — depicting him standing with Kirk's head in one hand and a tribble in the other. In bragging that Kirk's death would have a certain poetic justice to it, he revealed his plan, which led Captain Sisko and Jadzia Dax to search for the lethal tribble. They found it and beamed it into space where it safely exploded. Shortly after, Kirk exposed the younger Darvin, and history remained unchanged. The older Darvin was taken back to Federation space to face very serious charges.
As a merchant, "Barry Waddle" dealt mostly in gemstones, kivas and trillium. While on Cardassia, he got terribly sick of drinking hot fish juice every morning, and when he came onto the Defiant, he was relieved to finally get a taste of Raktajino again.
References: The Trouble with Tribbles,