Klunk was the evil counterpart of Clank whose homeworld was planet Mariona. He was created by Dr. Nefarious to be a spy replacing Clank. His name was given to him by Nefarious as a joke in Metropolis. Compared to Clank, he was as same looking and weighed as possible so Ratchet couldn't see any difference between them. Klunk had red eyes instead of green like Clank, but Klunk could turn them green which he did to fool Ratchet in Ratchet & Clank: Up Your Arsenal.
Klunk replaced Clank after his completion of an episode of Secret Agent Clank at Holostar Studios. As the true Clank refused to assist Dr. Nefarious, he was imprisoned and Klunk built to take his place. Klunk continued posing as Clank throughout a large portion of the remaining game, with Ratchet only extremely slowly becoming aware that something was amiss. Klunk's true identity was only revealed at the end of the Metropolis level, at which point he transformed into his Giant Klunk form and dueled Ratchet atop a pair of moving Hover Trains. Klunk was seemingly destroyed and the real Clank returned.
Klunk was later revealed to have survived his defeat in Up Your Arsenal. He had spent the next few months making hightech toilet seats but quit after the defeat of Otto Destruct. He was the mastermind in Secret Agent Clank, was also behind the theft of the Eye of Infinity and framed Ratchet by using a mind control device on him. He had controlled The Kingpin the whole time and created a laser that could blow up every planet in the Solana unless Clank stopped it.