Some Fall Out of Vogue. Gabriella Karefa-Johnson Jumped.

She has effectively become a test case for what it means to be a voice of fashion in a post-magazine world — a world where the power that was once concentrated in the hands of a few glossy publications is increasingly lodged in the feeds of charismatic individuals, where fashion itself has become part of pop culture, and where personal values are increasingly infiltrating the professional sphere.

But two things tend to happen when an umbilical cord is cut. Either you soar ever upward or you disappear into the chaotic maw of space.

Act I: The Serena Williams Effect

The story of Ms. Karefa-Johnson’s emergence as a public figure is essentially a morality play involving unintended consequences, social-justice reckoning and this peculiar social media moment.

“Basically, when I was an assistant, I would go on these shoots with celebrities and famous models, and we would become friends because I was normal and would treat them like normal people,” Ms. Karefa-Johnson said. She was talking about women like Bella and Gigi Hadid and Paloma Elsesser, who are part of her inner circle. “They would follow me on Instagram,” she said, and when she posted something they liked, they would repost it to their millions of followers.

As she spoke, she was wearing a Public Enemy “Fear of a Black Planet” T-shirt, jeans and Chanel ballet flats and was curled up on the sofa in the living room of the four-story brownstone in the Bedford-Stuyvesant section of Brooklyn that she shares with her twin sister, a rapper. She also has an older brother, who is in cybersecurity, and two older sisters. One is a lawyer who works for a social-justice nonprofit in California, where they all grew up, and one is in sports marketing.

Her family is very important to her, she said, especially her maternal grandparents. Her grandmother was the first Black woman to graduate from Yale Divinity School; her grandfather, a doctor, was the first foreign minister of Sierra Leone after it achieved independence from Britain and became a republic. They helped raise her and her siblings after her father, an urban planner, died when she was 7 months old. She pointed to her aunt, who was a model in Europe in the 1960s and ’70s, as the reason she first got interested in fashion.