The blue-and-yellow banners fluttering across South Korea showed a 17-year-old girl with gentle eyes and a neat bob, her smile frozen in time. The red letters beside her portrait cried out with an urgency that never dimmed for a quarter century.

“Please help me find Song Hye-hee!”

After she vanished on a winter night in 1999, her father, Song Gil-yong, made looking for her his life’s work. As he traveled the country putting up banners and replacing ones that had faded in the sun and rain, his face became deeply creased and tanned.

The banners, each roughly the length of a car, stretched across sidewalks as office workers hurried past. After dark, they caught the glow of streetlights and neon signs.

“He was always hopeful that she was out there somewhere,” said Na Joo-bong, 67, the chairman of a national organization for missing children in South Korea and one of Mr. Song’s closest confidants. “He had one wish: to hold her hand one day.”

The banners made Mr. Song a symbol of parental devotion in his country. But he paid enormous personal costs. His wife took her own life. His relationship with his eldest daughter splintered. His savings dwindled with each new banner he bought and each mile he drove in his small white truck.