Dug is a Golden dog and a member of a pack of dogs that have collars that allow them to talk and are owned by Charles F. Muntz, the childhood hero of Carl and Ellie Fredricksen. Dug differs from the other dogs in the pack in several ways. He is not tough looking as they are, and he is a light brown color, whereas most of them are darker colors. Aside from appearance, he also differs from them in a very important way: he has a very good heart.
Dug is not ferocious like his fellow dogs, and instead is very loyal and loving, especially to Carl and Russell. He does show that he can be brave and his loyalty really shines through when he protects Carl from Charles by lunging himself at Charles, even though he might get hurt. He also faces the head dog, Alpha, who has previously made Dug wear the Cone of Shame, which Dug has said that he does not like. He battles Alpha briefly and by accident makes Alpha wear the Cone of Shame, causing all of the other dogs to follow him and not Alpha.
Dug is also not very interested in Kevin or in capturing Kevin, but it is more as if they (the dogs and Charles) make him feel as if he has to. Before he met Carl and Russell, that is all that he knew that he had to do. Afterwards, he met Carl and Russell, whom he instantly loved. As a result, he remained loyal to them, even when Carl told him to leave. He stays and says, "I hid under your porch because I love you." He is truly the perfect definition of a loyal friend, one who truly loves.
Through the short Dug's Special Mission released with the dvd of Up, we learn that Dug was put on a special mission by Alpha and his pack to move him out of the picture while they looked for the bird. No matter what, Dug always came upon them again and he overhears Alpha telling Charles Muntz that Dug lost the bird. It is Dug's birthday that day, and he made a wish, but after this he becomes sad. He thinks he sees a man, and finally he sees Carl and Russell, and he knows that it is a good birthday after all, because he got his wish.
From the Official Website Production Notes