Gibber is the monkey which Captain Flint gives to Roger Walker as his reward for his part in finding the stolen manuscript of Mixed Moss. Unlike Polly, Titty's parrot, Gibber only appears in two "Peter Duck stories", specifically Peter Duck and Missee Lee although he is occasionally mentioned in some of the other books as spending time at the zoo while Roger is on holiday.
Gibber appears in the early incarnation of Peter Duck, 'Their Own Story' (published in CFT) – in this text his name oscillates between Gibber and Jacko.
Gibber as a plot device
Gibber's most significant role is when he sets fire to the Wild Cat in Missee Lee with Captain Flint's cigar. He did sign the Ship's Papers of Swallow, though we don't know when.
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You are standing into danger. This article or section contains conjecture.
It appears that Ransome realised that a real monkey camping in the Lake District would cause significant problems if he tried to include him as a participant in the "realistic" stories, hence Gibber's zoo holidays. He is not mentioned in most of the books and his absence is not usually commented on.
Pronunciation
Is it "Gibber" or "Jibber"? Roger said Gibber is so-named "because he gibbers"ref req. Wiktionary defines gibber: To talk unintelligibly or incoherently but gives no opening consonant guidance while Oxford plumps for 'J-' and Dictionary.com goes for 'G-' or 'J-'. The Japanese translators call Gibber Jibbaa.
Native source
Jacko is the name of E F Knight's monkey in his book The Cruise of the Alerte, which is a significant source for Crab Island's geography in Peter Duck'.