Soon, Brad caught the attention of Bruce Weber, who cast him in one of his notoriously racy Abercrombie & Fitch campaigns. “I was on the bag, which is kind of a big deal,” Brad said, meaning the store’s shopping bag. He was also in its catalog. “I was fully nude, like a butt shot.”

In 2001, Brad moved to New York and showed up at the Ford agency, where the founder, Eileen Ford, saw him from across the room and said, “Oh, my gosh, he looks like Errol Flynn,” according to Sam Doerfler, who became Brad’s agent. He walked out with a three-year contract. Doerfler said he thought that Brad had the kind of clean-cut athleticism that would appeal to more commercial clients (brands like Target or Macy’s), but Brad wanted to do high fashion; he wanted to work with Lagerfeld. “My thing was, how do you take a commercial-looking guy and make him look edgy,” Doerfler told me. To give Brad a more distinct look that might attract European designers, Doerfler had Brad spend a year growing his hair long and transforming his muscular physique into a more lanky one.

By 2003, Brad debuted on the Jil Sander and Dolce & Gabbana runways in Milan and quickly landed the covers of L’Uomo Vogue and Interview, a coup for a male model. That same year, VMAN flew Brad, 23, along with two other male models to Biarritz for a shoot for which Lagerfeld would be the photographer. The designer owned a hillside estate there at the time, and he suggested they shoot in the outdoor shower. The resulting image shows Brad fully nude, his right hand covered in tangles of Chrome Hearts chunky jewelry and grabbing his genitals. “He probably took one picture of each of the other guys and, like, 20 of me,” Brad said.

After Biarritz, Lagerfeld photographed Brad constantly, almost as if he were studying an exotic new species: Brad walking, sleeping, eating, shaving, swimming and working out; Brad nude or seminude in showers and bathtubs, on beds and on balconies. He dressed him up like his own Ken doll, shooting him as the Greek god Zeus, James Dean and Jay Gatsby. Lagerfeld compiled the photos in “Metamorphoses of an American,” a four-volume book devoted entirely to Brad. In the introduction, he wrote, “It’s all about the clarity of the transmitted individuality of a face and a body unencumbered by too much experience.”

Brad soon became known as Lagerfeld’s “muse.” Amanda Harlech, a socialite who has been a muse of Lagerfeld’s for almost 20 years, said, “At a very simple level, it’s something that the eye is pleased to look at.” Brad has always been comfortable in the role. “The photographer has to be into the subject he’s shooting,” he said. “It’s like if you’re a basketball coach, you have to be into LeBron James and think he’s great, or you wouldn’t put him in the game.” He added: “The models that are uncomfortable just don’t make it. Why not get naked in the shower and have million-dollar jewelry on me?”

Brad began to appear in Fendi and Chanel ads almost every season. In the past decade, he has been shot by Mario Testino, Patrick Demarchelier, Craig McDean and, months before his death, Richard Avedon. In 2004 Brad was named the top male model by Models.com, a site that releases rankings for the industry, and held the title for three years. Recently, Vogue listed him among the “Top 10 Male Models of All Time.”

Since Brad’s arrival, Lagerfeld’s entourage has grown to include the British model Jake Davies, 34, and Baptiste Giabiconi, a 25-year-old from the south of France with a striking resemblance to a younger Lagerfeld. Together, they’ve become a part of the designer’s provocative image, trailing him as he exits cars and boats and planes. When I asked Lagerfeld about his “boys,” he said: “I don’t give labels for it. Labels is something I design for, they’re not what I give to persons.” Then he relented. “I see them like family,” he said. “I have no family at all, so it’s good to have, like, sons but without the unpleasant problems sons can create.” He added: “It’s a choice, it’s not an obligation. There’s a big difference. I have a sister in America who I haven’t seen for 40 years. Her children never even send me a Christmas card.”