Comet is first seen on Scott Calvin's roof after the previous Santa fell. Initially, he and Scott did not get along, as he growls at him during their first meeting. By the next Christmas, Comet has come to like Scott and has even made a rope for him to use to save himself in case he falls off a roof.
Comet flies Scott down to Chicago to visit Charlie after he starts having problems at school. While visiting with the Millers, Comet stays in the backyard. When Scott is hanging some Christmas lights on the house, Comet tells him they are not straight. He is delighted when Lucy comes to see him and feeds him chocolate, which he is not allowed to have, as Santa mentions he tends to overeat. He gives Scott a look after he lies to Lucy about being Santa.
Later on, as Scott is trying to return to the North Pole because of the Toy Santa taking over the workshop, he goes out to the backyard to retrieve Comet. He has eaten too many sweets and become too fat to move. When Scott asks him if he ate all the sweets, he blames it on a squirrel. Scott tries moving him with no success. He is unable to take Scott and Curtis back to the North Pole, causing them to find an alternate method of return with the Tooth Fairy. Comet stays in the backyard until Christmas Eve when Scott comes to retrieve him after defeating the Toy Santa. As he is still unfit to fly the sleigh, Chet takes his place during the toy delivery, and he rests on the back of the sleigh throughout the journey.
Comet's appearance changes between the films.
Comet was played in some scenes by a real reindeer but was mostly animatronic, and occasionally animated.