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“The name's Jock! [...] Heather Lad O'Glencairn, to you!”
Although sweet and loyal, Jock can be the most aggressive dog in Lady's circle of friends, in spite of being the smallest. Nevertheless, he values his loved ones and makes an effort to make such a fact clear, on a consistent basis. He treats Lady as his own relative, watching her grow alongside Trusty and continuously congratulates the spaniel on her many accomplishments, such as the day she received her first collar. He also takes it upon himself to mentor the young pooch, teaching her the ways of welcoming a new baby into the household and telling her stories of Trusty's past and such.
He is notably the best friend of his neighbor, Trusty who, like Lady, he treats like a relative, along with the lines of a brother. The two get along fine, but with Trusty's bumbling ways and Jock's occasional aggression, they sometimes clash, though they never let it tamper their friendship for long, if at all.
Though he is essentially warm-hearted, Jock tends to be stingy at times, and never lets anyone see his prized collection of hoarded bones, even Lady, and he frequently praises things based on their expense, such as Lady's collar and the coming baby. This trait is a reference to the stereotype of Scots being frugal.
As seen in the film, some of Jock's dislikes include the exposure of his prized bones, Trusty's clumsiness and "mongrels" like the Tramp.
In spite of his size, Jock is a fearless protector and will go to great lengths to protect the ones he cares about. This is shown when he threatens to attack Tramp (a dog twice his size) if he continues to "harass" Lady, and lastly when he risks his life to stop the dogcatcher's wagon from taking Tramp to the pound, as a way of apologizing to Tramp for mistreating him early on in the film, which shows he's honorable and admits to making mistakes.
Jock first appears while singing his song as he buries his bone in his secret spot, along with many other bones. Jock is then encountered by Lady who presents her brand new collar complete with a license. They then go to visit Trusty, who is dreaming. Jock confesses to Lady that Trusty has lost his sense of smell. The next day, Jock and Trusty visits Lady, as it is a perfect day until Lady tells them bad news. She tells the duo that Darling has not been treating her the same, along with Jim Dear.
Jock and Trusty quickly realize what's going on and as Jock puts it, "Darling is expecting a wee bairn." They start to describe what a baby is until Tramp arrives. Tramp shoves Jock out of the way to speak to Lady, which upsets Jock, who barks at him to keep away from Lady.
Jock later reappears near the end of the film after Lady and Tramp got to know each other. At this point, Lady cannot stand him. And when Tramp returns after being separated by the dogcatcher, Jock is prepared to attack when ready, although Lady claims it's not necessary. After Tramp redeems himself by killing the attacking rat, Jock and Trusty realize that Tramp is really not all that bad for a dog. Jock felt very ashamed for misjudging Tramp badly from beginning to end.
To redeem themselves, Trusty and Jock rush to the dogcatcher's wagon and attempt to stop it. They succeed in saving Tramp, but unfortunately, Trusty is run over when the wagon crashes. Jock nudges his best friend, but Trusty does not move, leading Jock to howl in mourning. At Christmas, Jock joins Lady and Tramp, who have mated with four puppies. Trusty survived the crash with only a broken leg and joins the celebration as well. As the feisty Scamp unravels part of his plaid sweater, Jock congratulates Lady and Tramp on their puppies, also noting Tramp's collar.
Jock makes another cameo in the spin-off TV series in the pilot episode, "Home is Where the Bark Is". When Lucky asks Jock for a ride in his human's boat, after they miss the ferry, he at first turns them down before realizing that they are the puppies that Pongo and Perdita sent out a barking chain message about. He is then all too happy to use to boat to help the pups cross the water (his human is asleep during this sequence).
Later, as the pups race Cruella back to the Dearly Farm, Rolly frets that there is no way they can outrun Cruella's car. Jock then tells the pups to get going as he sends out another barking message to the farm animals so they can set up traps to delay Cruella. In the series, he is shown to wear a red sweater which he is seen wearing at the end of the original film.
Jock reappears in the sequel. He and Tusty remain close to Lady and Tramp, being known as "Uncle Jock" to their puppies, Annette, Collette, and Danielle. He and Trusty are first seen as they watch happily while Lady, Tramp and their family pass through during a stroll. They later appear after Scamp ran off and the news is brought to their attention by Annette, Collette, and Danielle. Jock joins Lady, Trusty, and Tramp as they search for Scamp. Trusty and Jock continue to search and are eventually reunited with Scamp and join the family as they welcome Angel to the family.
Jock appears as a female in the film remake, voiced by Ashley Jensen in the remake. Although she is called "Jock" in this version, her owner also calls her "Jacqueline" in this film. Unlike in the original film, this incarnation of Jock has no solo song she sings at any point of the film.
Jock is first seen talking with Lady and Trusty where she feels overjoyed upon seeing Lady's collar just as she tells her friends that she is part of Jim Dear's family. Just as Trusty decides to find the rat, Jock's owner takes her back to her so that her owner can continue more paintings related to Jock. Having a conversation with Tramp, Jock and Trusty arrive just as Jock tells Lady to stay away from him just as Tramp walks away while she and Trusty are unsure of what Lady would do when a baby arrives in their house.
Later when Lady is reunited with Jim Dear's family, Jock and Trusty are happy for having Lady back with her friends where she explains that Lady has Lulu in her owners' family to which Jock finds it exhausting while Trusty feels overjoyed that she stayed away from Tramp thinking Tramp is a nuisance. Jock then explains that life as a street dog is extremely bizarre for her while her owner tells her to go back to her while Trusty feels overjoyed that Lady is back to talk to him again. Later when Tramp is being sent to the dog pound, Jock also comes to the rescue with Lady alongside Trusty in order to rescue Tramp from being impounded. Trusty then leads Lady and Jock to the scent he found to which he found the carriage leading to the dog pound containing Tramp inside. Having saved Tramp from the dogcatcher and when Christmas arrives, Jock then meets the street dogs Tramp encountered while Trusty decides to tell the story about of how he saved Old Reliable from the dogcatcher while Jock and the stray puppies listen to his story during Christmas.