Clifford is a male red dog whose appearance, disposition, and behavior are based on a giant "all-around" dog.

Clifford is shy, gentle, friendly, loyal, lovable, clumsy, well-meaning and helpful. He sometimes gets into trouble because of his size or is tempted into trouble by his friends and those he meets. His size, like giants in medieval legends, is inconsistent—he is often shown as being about 25 feet tall from paws to head, but can appear far larger: For example, in one episode of the show, he removes the top of a lighthouse and swims out with it to guide a ferry through the fog to the dock.

The character's name is based on the imaginary childhood friend of creator Norman Birdwell's wife. He originally wanted to name him "Tiny", but his wife persuaded him otherwise. His owner is Emily Elizabeth, to whom he is devoted. He has a mother, two brothers, and two sisters. His character was created when an editor at Harper & Row advised Birdwell to write a story to go along with one of his pictures. She picked out his sketch of a baby girl and a horse-sized bloodhound and casually said, "There might be a story in this," Birdwell remembers.