Cy-Kill had served as a Guardian during the first Renegade revolt, and in recognition of his achievements was made on of Gobotron's three High Protectors. The Guardian Council tasked him with moping up the remnants of the Renegade movement. However, Cy-Kill underestimated his enemy, and was nearly killed by Zero. Upon his return he was angered by the Guardian Council's response to this failure, and became convinced that the only way to make Gobotron strong was by ruling it himself. After an attempt to persuade comrade Leader-1 to join him in taking control of the planet, but he was turned down. Instead he defected to the Renegades, along with Fitor and Dart.
His involvement reinvigorated the Renegades, who united under him. Cy-Kill installed Fitor as his second-in-command and constructed a Fortress on Gobotron. Later, he made contact with the human Zebediah Braxis, who tried to help the Renegades take over Earth. Cy-Kill used the planet to make a vast army of Zods with which to invade Gobotron. He was foiled by Leader-1 and the Guardians, while the Fortress was stormed and destroyed. Cy-Kill and most of his best troops were captured and imprisoned on Gobotron's Prison Moon.
However, he soon escaped, freeing his troops and heading to his flagship Roguestar. The return of the Master Renegade made a full-scale attack on Gobotron viable, and the upgraded fleet easily swept the Guardians' defence forces aside. Leader-1 was shot down and disassembled, but was repaired by the Last Engineer and led the remaining Guardians on a counterattack from Earth. The Master Renegade made a grab for power in the middle of the Guardian attack, temporarily causing the Renegade defences to fail. Cy-Kill recovered and imprisoned his predecessor, but it was too late and the Guardian Power Warrior Courageous forced the Renegades to retreat from Gobotron.
Cy-Kill would then divide his time between various schemes to invade Earth and/or Gobotron. He managed to take control of America with the help of the crimelord Murchison, but was foiled by Matt Hunter and Flip Top. Cy-Kill would also briefly take control of Gobotron by holding the planet's population to ransom, but was outwitted by Leader-1 and driven off. His plans varied from trying to undermine the Guardians' image to threatening Earth with a Nova Beam to trying to resurrect ancient Gobot the Evil One, but all tended to have the same outcome - failure, either due to Guardian intervention, or the bumbling of his own troops (usually Cop-Tur's).
Despite these constant failures, Cy-Kill largely held the respect of his troops. One exception was Zero, who challenged Cy-Kill after another failed raid on Gobotron. He was beaten in combat, but eventually managed to avert a split in the ranks. Cy-Kill tended to operate with Crasher and Cop-Tur, and trusted Fitor to effectively run Roguestar whenever he was elsewhere. He was recaptured by the Guardians at one point, but used his mind control powers to engineer his escape from Elba.
His genuine friendship with Fitor is one of several indications that Cy-Kill isn't perhaps a complete monster - he also joined the Guardians in preventing the return of the Evil One when he realised the full implication of what it would mean. He also (initially at least) seemed to genuinely believe that Gobotron would benefit from strong leadership.
Cy-Kill appeared in the following episodes:
"The Battle for Gobotron", "Target Earth", "Conquest of Earth", "Earthbound", "The Final Conflict", "Cy-Kill's Escape", "Quest for the Creator", "The Fall of Gobotron", "Flight to Earth", "Return to Gobotron", "Time Wars", "Renegade Alliance", "Genius and Son", "Dawn World", "Forced Alliance", "Invasion from the 21st Level Part I", "Invasion from the 21st Level Part II", "Lost on Gobotron", "Cy-Kill's Shrinking Ray", "Quest for Roguestar", "Renegade Rampage Part I", "Renegade Rampage Part II", "Ultra Zod", "Sentinel", "Cold Spell", "Auto-Madic", "Tarnished Image", "In Search of Ancient Gobonauts", "Gameworld", "Wolf in the Fold", "Depth Charge", "Nova Beam", "Transfer Point", "Steamer's Defection", "The Gobot Who Cried Renegade", "The Seer", "Whiz Kid", "Pacific Overtures", "Destroy All Guardians", "Escape from Elba", "Clutch of Doom", "The Third Column", "A New Suit for Leader-1", "Renegade Carnival", "The Gift", "The Last Magic Man", "Braxis Goes Bonkers", "Inside Job", "Fitor to the Finish", "Cy-Kill's Cataclysmic Trap", "Doppelganger", "Trident's Triple Threat", "Speed is of the Essence", "Crime Wave", "Scooter Enhanced", "Ring of Fire", "Element of Danger", "It's the Thought That Counts", "Guardian Academy", "Quest for New Earth", "The Secret of Halley's Comet", "Terror in Atlantis", "Et Tu, Cy-Kill", "Mission Gobotron", Gobots: Battle of the Rock Lords