Jonner Jin, a male human, was a member of Iron Squadron, a group of rebels operating from the world of Mykapo. In the year 2 BBY, Jonner and his fellow insurgents found themselves in a skirmish against an Imperial scouting party. They received assistance from the Spectres and Phoenix Cell, who had arrived to help evacuate civilians from Mykapo. Initially, Iron Squadron declined the Spectres' suggestion to leave the system before Imperial reinforcements arrived. However, Jonner, along with Gooti Terez and R3-A3, recognized the danger and departed with the Spectres. Their captain, Mart Mattin, chose to stay behind and confront the Imperials. Jonner, Gooti, R3-A3, and the Spectres later collaborated on a successful mission to rescue Mart. Following this, Iron Squadron formally joined the rebellion.
During the Age of the Empire, Jonner Jin became a part of Iron Squadron. This rebel cell, based on Mykapo, comprised the humans Mart Mattin, Mart's father, the Theelin Gooti Terez, and the astromech droid R3-A3. The Sato's Hammer, a YT-2400 light freighter, served as Iron Squadron's base of operations. After Mart's father passed away, Mart assumed leadership of the group. Mart did not engage with the broader rebellion and did not maintain contact with his uncle, Commander Jun Sato, who led the Phoenix Cell. Iron Squadron focused on harassing Imperial forces within their system.
In 2 BBY, the Galactic Empire declared martial law on Mykapo. Iron Squadron engaged with an Imperial scouting force. During this skirmish, the hyperdrive of Iron Squadron's ship was damaged. Shortly afterward, the Spectres and Phoenix Squadron appeared to evacuate rebel sympathizers from Mykapo. The Ghost, the Spectres' ship, successfully destroyed the TIE fighters that were attacking the Sato's Hammer. This allowed Iron Squadron to use explosives to destroy the Imperial Gozanti-class cruiser. After the skirmish, Mart invited Hera Syndulla, the leader of the Spectres, and her crew to meet with Iron Squadron.
Jonner and Gooti welcomed Hera and her team, consisting of Ezra Bridger, Sabine Wren, and the astromech droid Chopper, onto their ship. When Sabine commented on the freighter's poor condition, Jonner replied that they did not require a hyperdrive. He led the Spectres to their captain, Mart, who was in the cockpit. Jonner supported his captain's objection to Hera's advice that they leave the system before Imperial reinforcements arrived. He even sarcastically questioned the meaning of "evacuate," with Mart adding that it meant running away like a coward. He also showed an unwillingness to heed Hera's arguments. With the two groups unable to reach an agreement, Hera and her crew departed the Sato's Hammer and returned to the Ghost. Ezra persuaded Hera to allow him, Sabine, and Chopper to remain behind to repair Iron Squadron's hyperdrive in an attempt to win them over. Hera agreed, but warned them that they had to leave on the shuttle Phantom II at the first indication of Imperial activity.
Jonner and Gooti observed as R3-A3, Sabine, and Chopper worked on repairing the hyperdrive. Mart considered it a waste of time and returned to the cockpit. When Ezra informed Jonner and Gooti that they needed to be ready to leave if they could not fix the hyperdrive, the two Iron Squadron members expressed their reluctance to abandon their captain and their home. Ezra responded by saying that they had to choose their battles carefully. R3, Sabine, and Chopper succeeded in repairing the hyperdrive, but their efforts were negated when Mart diverted the power to the ship's laser cannons and deflector shields. Jonner and the other rebels soon discovered that Imperial reinforcements under Admiral Kassius Konstantine had emerged from hyperspace. Upon seeing Konstantine's forces, Jonner and Gooti realized they were outmatched.
He and Gooti attempted to reason with Mart, but their captain was determined to fight the Empire. After the ship's power failed, Mart ordered an evacuation. Jonner, Gooti, R3-A3, and the Spectres escaped aboard the Phantom II, but Mart changed his mind and decided to fight Konstantine's forces. Konstantine easily defused Mart's bombs and disabled the Sato's Hammer's engines. Jonner and the other rebels were forced to leave Mart behind and fled into hyperspace. After meeting up with the rebel fleet, Jonner attended a briefing with Hera, the Spectres, Commander Sato, and Captain Rex. Sato authorized a mission, led by Hera, to rescue Mart. Jonner and Gooti were overjoyed that they were going back to save their captain.
During the rescue operation, Jonner and Gooti were stationed in the cargo bay of the Ghost. They opened the cargo bay doors, allowing R3-A3 and Chopper to use their rocket boosters to disable a magno-mine that had been placed on the Sato's Hammer. Following Hera's instructions, they attached the Ghost to Sato's Hammer. After retrieving Sato's Hammer, the rebels were reinforced by Commander Sato, who arrived with a CR90 corvette and several A-wings. Before jumping into hyperspace, the rebels dropped the magno-mine onto Konstantine's light cruiser, severely damaging the Imperial ship. Jonner then witnessed Grand Admiral Thrawn's Imperial Star Destroyer Chimaera emerge from hyperspace. Jonner and his comrades recognized the immense size and power of an Imperial Star Destroyer and understood that it was foolish to confront one alone.
Jonner, Iron Squadron, and their rebel allies then journeyed to the Phoenix cell's Chopper Base on the planet Atollon. There, Jonner and Gooti embraced their captain after he was reunited with his uncle, Commander Sato.
Jonner Jin was a bald, somewhat overweight male human with blue eyes. He served as the muscle of Iron Squadron and was extremely loyal to his friends, Mart Mattin and Gooti Terez. He demonstrated that he was more open-minded than his captain, recognizing the validity of Ezra's argument when Ezra attempted to persuade them to leave the system. Jonner was a cheerful and friendly person, often smiling and offering the Spectres space waffles when they were on board their ship.
In "Iron Squadron," the eighth episode of the third season of Star Wars Rebels, Jonner was voiced by Eric Lopez. He was named in honor of John Harper, a cameraman, editor, and producer for The Star Wars Show.