One of the stylization choices made for Up was that a square represented the past and a circle represented the future, the reason for Carl being box-shaped. After the death of his wife Ellie, Carl has shut off the world around him and has sunk very low to the ground, like a brick. But the characters around him, which have curves, circles and other shapes making up their figures, continue to change while Carl stays within the confinements of his "square".
Following Ellie's death, Carl became lonely, cranky, and bitter and misses his wife terribly, but became nicer with Russell's help.
Carl has the typical stubborn character of a man his age but deep down he's a kind person, but has his limits in patience and temper. He found Russell, Dug, and Kevin to be annoyances and obstructions in his goal to fulfill Ellie's dream of moving their home to Paradise Falls, but after he read her last message to him, he did what was right and saved Russell and Kevin, and accepted Dug as his dog.
When he lost his and Ellie's house, he was saddened but realized Ellie would have done the same, stating "It's just a house".
Carl vouched for Russell at his ceremony and gave the young scout the very same soda bottle cap that Ellie gave him the day they met, having come to love Russell as the son, or perhaps grandson he never had.
In the late 1930s, young Carl Fredricksen was a shy, quiet boy who idolized renowned explorer Charles F. Muntz. One day, Carl befriended a tomboy named Ellie, who was also a Muntz fan. She confided to Carl her desire to move her "clubhouse" — an abandoned house in the neighborhood — to a cliff overlooking Paradise Falls and made him promise to help her. Carl and Ellie eventually got married and grew old together in the restored house, working in a zoo as a balloon vendor and a zookeeper, respectively.
After a miscarriage, they repeatedly pooled their savings for a trip to Paradise Falls, but always ended up spending it on more pressing needs.
Just as Carl and Ellie, who were both senior citizens, finally seemed able to take their trip, Ellie contracted an illness and died of old age, leaving Carl by himself, becoming bitter and cranky and missing his wife terribly. As the years passed, the city grew around Carl's old house with construction as he refused to move.
After confronting a construction worker over his broken mailbox, the court orders Carl to move into the Shady Oaks Retirement Home. Carl comes up with a scheme to keep his promise to Ellie: he uses his old professional supplies to create a makeshift airship, using 10,000 helium balloons, which lifts his house off its foundations. Russell, a young Wilderness Explorer (a fictional scouting organization), becomes an accidental passenger in an effort to earn his final merit badge for assisting the elderly.
After surviving a thunderstorm, the house lands near a ravine facing Paradise Falls. Carl and Russell harness themselves to the still-buoyant house and begin to walk it around the ravine, hoping to reach the falls before the balloons deflate. They later befriend a tall, colorful flightless bird whom Russell names "Kevin" who is trying to reach her chicks and a dog named Dug, who wears a special collar that allows him to speak.
Carl and Russell encounter a pack of dogs led by Alpha and are taken to Dug's master, who turns out to be an elderly Charles Muntz. Muntz invites Carl and Russell aboard his dirigible, where he explains that he has spent the years since his disgrace searching Paradise Falls for the giant bird. When Russell innocuously notes the bird's similarity to Kevin, Muntz becomes hostile, prompting the pair to flee with Kevin and Dug. Muntz eventually catches up with them and starts a fire beneath Carl's house, forcing Carl to choose between saving it or Kevin. Carl rushes to put out the fire, allowing Muntz to take the bird. Carl and Russell eventually reach the falls, but Russell is angry with Carl over his decision to save his house instead of Kevin.
Settling into his home, Carl looks through Ellie's childhood scrapbook, seeing that it actually has many more photos of their lives in it than he thought, and finds a note from Ellie thanking him for the "adventure" and encouraging him to go on a new one. Reinvigorated, he goes to find Russell, only to see him sailing off on some balloons to save Kevin. Carl empties the house of furniture and possessions and pursues him.
Russell is captured by Muntz, but Carl manages to board the dirigible in flight and free both Russell and Kevin, having a fight with Muntz. Muntz pursues them around the airship, finally cornering Dug, Kevin, and Russell inside Carl's tethered house. Carl lures Kevin out through a window and back onto the airship with Dug and Russell clinging to her back, just as Muntz is about to close in. Muntz leaps after them, only to snag his foot on some balloon lines and fall to his death. Snapped from its tether, the house descends out of sight through the clouds, which Carl accepts as being for the best.
Carl and Russell reunite Kevin with her chicks, then fly the dirigible back to the city. When Russell's father misses his son's Senior Explorer ceremony, Carl presents Russell with his final badge: the grape soda cap that Ellie gave to Carl when they first met, to the applause of who appears to be Russell's mother in the audience. The two then enjoy some ice cream together, sitting on the curb outside the shop as Russell and his father used to do, with the dirigible parked nearby. Meanwhile, Carl's house is shown to have landed on the cliff beside Paradise Falls, as promised to Ellie.
Carl and Russell only appear at the end of the short film where Dug is looking at a rock shaped like a turtle during the events where he and Russell arrive in the rocky area of Paradise Falls. After hearing the voice, Dug reunites with the two to find a new master for him after Alpha, Beta, and Gamma scold Dug for not capturing Kevin, the rare bird Muntz was to capture, as he finds two new masters to assist him. During the image where Dug speaks to Carl and Russell, the two are suddenly surprised that he is able to talk before the image cuts to black.
Carl first appears telling nurses George and A.J. that he is ready to leave for his retirement at Shady Oaks. However, the two nurses notice there are lots of balloons on his house which floats over them as Carl bids them a farewell. The next day during morning news, a journalist tells the nurses to tell the public what they saw after mentioning that Carl was accused for harming a construction worker and now takes off with his house filled with balloons and indeed escaped.
Later during the events when Carl and Russell return with the Spirit of Adventure dirigible, he and Russell leave the dirigible while Russell tells him he would like to steer the dirigible's helm the next time they arrive.
- If Carl is 78 years old in 2009, and if he was 9 in 1939, he was either born in 1930 or 1929.
- Carl wore the same style of glasses at every age.
- Carl's personality may be a reference to Shel Silverstein's The Giving Tree.
- To lift a house such as Carl's (and rip it loose from the utility pipes/cables anchoring it) would, in reality, require vastly more balloons than are shown.
- An unpowered airship (such as Carl's house) would not be steerable; it would go wherever the wind goes.
- Both these last two points were Pixar deciding to go for believable rather than realistic.
- Although Carl doesn't have his own musical theme, Ellie's theme plays whenever he thinks about her, and also plays against Muntz's theme during the climax.
- Carl is the second human protagonist in a Pixar feature film, after Mr. Incredible from The Incredibles. He is followed by Merida from Brave and Miguel Rivera from Coco. He will be followed by Elastigirl in Incredibles 2.
- Carl is the second oldest protagonist of a Pixar film, the oldest being WALL-E, as he has been "alive" for more than 700 years (the movie WALL-E takes place in 2805, and Up takes place in 2009 the year it was released).
- Carl is also the second Pixar protagonist to lose his love interest after Marlin from Finding Nemo.
- In an interview with the director of Up, Pete Docter, he has stated that the letter Andy has addressed from Carl and Ellie in Toy Story 3, was signed to person by the name of Emma Jean who is Carl's previous love interest before Ellie but are now close friends who frequently go out together.