Mr. Trump takes credit for his children’s sunny devotion. “I’ve always been a very good father,” he told Anderson Cooper in a Trump family interview this summer on CNN. “They come to me, friends of mine, very successful people, and their children have problems with drugs and problems with alcohol and problems with a lot of things, and they say: ‘Could you speak to my son? Could you speak to my daughter?’ And I’m always very honored to do that.”

When asked to describe his youngest daughter, Mr. Trump replied by email: “Tiffany is a tremendous young woman with a big and beautiful heart. She was always a great student and a very popular person no matter where she went. I am incredibly proud of Tiffany and how well she has done. — DJT.”

(Tiffany did not consent to be interviewed for this article, although she did pose for its photo shoot. Instead, the campaign delivered a list of approved contacts. Other family friends who were not on the list said they were instructed not to speak without authorization.)

Outside acquaintances echo that she is more serious than she looks on social media. “She didn’t seem like a party girl at all,” Carson Griffith, a writer who followed the Snap Pack around the Hamptons in 2015 for Du Jour magazine. (Ms. Griffith also occasionally writes for The New York Times.) “She stayed in the city Friday night to finish a paper, and I never saw her drink a glass of wine.” Ms. Griffith was impressed by Tiffany’s good manners: “When I would ask about her, she would say: ‘How about you? What’s your job like?’”

Tiffany’s vacations were mostly spent on trips with her mother — a mix of fun mother-daughter getaways and good-will tours overseas, like handing out vitamin C pops at an orphanage in Malawi. A bodyguard went with her on visits to her Georgia relatives, but otherwise she blended easily in her mother’s small hometown. Sometimes, her celebrity status poked through.

Recalling a party for 3-year-olds, Janice Kiker, a close family friend in Dalton, Ga., said: “Tiffany showed up in a faded, worn-out tutu. I was shocked.” She added with a laugh: “Then someone told me it had belonged to Shirley Temple. I said, ‘Never mind.’”