Dave Thomas, an orphan who went to work in restaurants at 12 and by dint of hard work, quality control and shrewd marketing made Wendy's the third-largest hamburger chain in the world, died yesterday. He was 69 and was known to millions as the folksy character who made a low-key personal pitch for his own fast foods in more than 800 television commercials.

He died of liver cancer at his home in Fort Lauderdale, Fla., according to the company he founded, Wendy's International. He had been undergoing kidney dialysis since early 2001 and had quadruple heart bypass surgery in December 1996.

Mr. Thomas was adopted as a baby, but he lost his adoptive mother when he was 5 and two stepmothers before he was 10. His closest relationship was with his grandmother, with whom he spent summers. He said she had taught him the values he strove to build into Wendy's, particularly her oft-repeated admonition, ''Don't cut corners.''

Hence, the square shape of Wendy's hamburgers.

Mr. Thomas became a leading champion for the adoption of foster children, arguing that they needed love in their lives. President George Bush named him as a national spokesman on adoption issues, and when President Clinton signed a bill in 1996 to give parents who adopt children a tax credit, he thanked Mr. Thomas for his leadership.