Their numbers have reached 7.8 billion if not more, roughly the population of the Earth. Their ranks include police officers and firefighters, pirates and knights, astronauts and elephant keepers.

Their physical dexterity is limited, their facial features rather plain. But for more than 40 years, standing only four Lego blocks tall, they have been giants of the toy world and the object of untold hours of enjoyment for generations of children and collectors.

They are Lego minifigures, and their creator, the Danish Lego designer Jens Nygaard Knudsen, has died at 78. The Lego Group announced his death, describing him in a statement as “a true visionary whose ideas brought joy and inspiration to millions of builders around the world.”

The Lego company was founded in 1932 by Ole Kirk Kristiansen, a master carpenter, in the central Danish town of Billund, where he made stepladders, ironing boards, stools and wooden toys. It was christened Lego two years later, according to a company history — a name that combined the Danish words “leg godt,” meaning “play well.”

Over the decades, the company honed modern techniques of manufacturing plastic toys, patenting its signature stud-and-tube locking system for its building bricks in 1958. But until Mr. Knudsen’s innovations in the 1970s, Lego lacked a human or even humanoid element to enliven its playscapes.

“There was something missing from the houses, cars, planes and fantasy world these children spent hours playing with,” Sarah Herman wrote in her book “A Million Little Bricks: The Unofficial Illustrated History of the LEGO Phenomenon.” Mr. Knudsen’s minifigures, she wrote, went “on to define and drive” the Lego system “more than any other part since the launch of the new Lego brick in 1958.”

In 1974, the company introduced human figures best remembered for their appearance in the popular “Family” set, which included a mother and a father, a grandmother and two children, all with round yellow heads. The characters proved popular with young Lego enthusiasts but were too large to be comfortably employed in the small-scale Lego world.

Mr. Knudsen, who had joined Lego in 1968 and ultimately became the company’s chief designer, was tasked with overseeing the development of a new line of miniature figures. The project took him and the company through dozens of iterations, including the faceless “Extra,” which had stiff arms and no means of ambulation.

It was a start, but Mr. Knudsen wanted a character with greater capacity for play. Introduced in 1978, the blocky minifigure had movable arms and legs, C-shaped hands to grip other Lego elements, and basic if sometimes inscrutable facial features.

With its head made from yellow plastic, the minifigure had “no obvious ethnicity,” according to the company’s description. (Future characters featured natural skin tones.) Minifigures were people in the most rudimentary form, allowing seemingly infinite possibilities for children to imagine the lives behind them.

“As a child, when you’re creating and building an imaginative world from Lego, being able to play with real people in that world is part of the pleasure and fascination and gives character and life to whatever you’ve created,” Herman said in an interview. “They have so much spirit.”

Early minifigures included a police officer, a firefighter, a doctor, a gas station attendant, a knight and an astronaut. The line proved so popular that it grew over the years to include 8,000 characters, among them figures from the Star Wars and Harry Potter franchises. Lego-loving children who grow into Lego-loving adults have been known to place bride-and-groom minifigures atop wedding cakes.

Mr. Knudsen also was credited with designing Lego sets including the “Castle” theme, which Herman described as “one of the first to take Lego building to another time period.”

“The addition of these knights and guards,” she observed, “with their helmets, horses, and weaponry, transported the castle model from historical relic to a living, breathing battlement.”

Other sets that Mr. Knudsen helped design included the “Space” theme, introduced in 1978 — an undertaking Mr. Knudsen said sapped him of 14 months of work hours in a single year. Another popular set was the “Pirates” theme, which required a new look for the minifigures previously known for their anodyne expressions.

“It was necessary to alter the minifigure’s expression in order to develop credible pirates,” Mr. Knudsen said, according to the company. “A real pirate captain must have a patch on his eye, a peg leg and a hook!”

Little information was available about Mr. Knudsen’s early life. He designed cars, fire stations and a police heliport for Lego, according to Herman’s book, before his work on minifigures. He retired in 2000.

The Lego Group was not able to immediately confirm details of Mr. Knudsen’s death. A colleague, Lego designer Niels Milan Pedersen, told Agence France-Presse that he died Feb. 19 at a hospice center in Hvide Sande, Denmark, and that he had amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease.

Survivors, according to the AFP report, include his wife, Marianne Nygaard Knudsen.

Some children who grew up playing with Legos retained such an affection for their toys that they became collectors. Many more became parents of children who delighted in the stubby bricks and minifigures, leaving a trail of them across the house, with others still to turn up under car seats and in coat pockets.

“I am convinced,” Mr. Knudsen told Herman, “that the minifigure will live as long as children play with Lego.”