Her bedroom at home is pale pink. Over her bureau is a painting of a pink cassette tape that she bought in a market for about $20. Her boyfriend hates it, she said. She has been dating Arthur Donald, Paul McCartney’s grandson, for almost two years. They met when she and Ms. Kianni did a collaboration with Stella McCartney, his aunt. He lives in California and tries to come to New York for weekends. When he visits, she said, he takes the painting off the wall. They are looking for something to replace it.

She is very online: She has almost 500,000 Instagram followers, to whom she posts photos of her activism and black-tie awards nights with Mr. Donald, like the Albie Awards of the Clooney Foundation for Justice, and about 242,000 TikTok followers. One of her most popular TikToks involved a bubble tea showdown with her father. They have an ongoing argument about texting. He likes email; she does not. (Now he texts her when he has sent her an email.)

They share, however, an appetite for what he calls “risk.”

Rewriting the Stanford Narrative

The idea for Phia, a portmanteau of Phoebe and Sophia, started with Ms. Gates (who once thought she might go into women’s health, the focus of her philanthropy) and Ms. Kianni (who wanted to be an environmental lawyer) trying to come up with a pitch to get into an entrepreneurship class.

First, they thought of a Bluetooth-smart tampon that would know what was going on with your hormones, iron level and so on. They considered making “the Gen Z version of LinkedIn.” Then, they thought about what so many women who started their own fashion labels had thought about: their own experience.

Ms. Gates remembered seeing an Area dress she had bought for $500 reselling for $150 on the RealReal and feeling, she said, “so foolish.” Ms. Kianni, who is Iranian American, grew up in Washington, D.C. and started a climate change organization, Climate Cardinals, when she was in high school (it translates climate resources into 100 languages); she was already a dedicated resale shopper.

They thought there had to be others like them — you know, Ms. Gates said, “smart girls, age 25 to 30, who want to shop like a genius and get the best price in one click.” They were so excited about the idea that they wanted to drop out and get started right away, but their mothers stepped in.