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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 includes 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. Three instances of this can be seen, where 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 repaying the mutt for mistreating him early on in the film; showing he's an honorable dog that is not afraid to admit mistakes.
Jock first appears while singing "Jock's Song" as he buries his bone in his secret spot, along with many others. Jock is then encountered by Lady who presents her brand new collar complete with a license. They then go to visit a dreaming Trusty. As Trusty dreams, Jock confesses to Lady that unfortunately Trusty has lost his sense of smell. The next day Jock along with 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. Jock 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 alright. After Tramp redeems himself by killing the attacking rat, Jock, along with Trusty, has seen that Tramp is really not all that bad for a dog. Jock felt extremely stupid and ashamed for misjudging Tramp badly.
Jock and Trusty rush to the dogcatcher's vehicle and attempt to stop it. Although they do, Trusty is apparently killed in the crash. Jock attempts to help, but it seems there is no use. Jock then howls and mourns for his seemingly departed friend. That Christmas, Jock joins Lady along with her husband Tramp and four puppies. Despite Scamp, Lady's only son, tearing up his plaid sweater, he remains calm and congratulates Lady and Tramp on their kids, also noting Tramp's collar. As for Trusty, it seems he survived the crash with a broken leg, and is shown attending the Christmas celebration as well.
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.
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. Jock and Trusty continue to search and are eventually reunited with Scamp and join the family as they welcome Angel to the family.