In May 2015, The New Yorker published a profile of the Silicon Valley investor Marc Andreessen. In it, writer Tad Friend joined Andreessen in his living room to watch an episode of Halt & Catch Fire, the AMC drama chronicling the rise of personal computing in the early 1980s. The scene provided an intimate window into the billionaire’s home life. Friend described a powder room toilet so opulent it wasn’t immediately clear how to flush it; the rooms were grand to accommodate Andreessen’s gigantic presence. Friend chronicled the endearing flourish with which the investor’s wife presented dinner—omelettes and Thai salads for two, served on Costco TV trays. Andreessen’s obsession with a punk software prodigy shed light on his self-conception as a man aligned with the industry’s outsiders.

There was one presence Friend failed to document. That would be Margit Wennmachers, who spent the evening tucked on the couch across from Andreessen and his wife.

An operating partner at Andreessen Horowitz, Wennmachers is among the most skilled spin masters in Silicon Valley. She has a sixth sense for communications strategy, which has helped her educate the world about the revolution technology is powering. She knows how to create the memorable scene that will shape a story. She understands how to get ahead of bad news that’s about to break and when to push startup founders to take responsibility for their actions. She returns nearly every call within 30 minutes, be it from a blogger, portfolio company CEO, or New York Times reporter. Over the past two and a half decades, Wennmachers, 53, has worked with, advised, or broken bread with nearly everyone who has endeavored to build—or write about—a startup. “She’s like the router at the center of the industry,” Andreessen says.

In many ways Wennmachers is an architect of Andreessen Horowitz, the prestigious investment firm that has backed hundreds of startups, including Facebook, Airbnb, and Twitter. Or, at least, she’s the architect of what the firm appears to be—and her presence has left an indelible imprint on the hundreds of businesses that have come into contact with the firm. Because of her, Silicon Valley looks very different than it did even a decade ago.

Over the past two and a half decades, Wennmachers has worked with, advised, or broken bread with nearly everyone who has endeavored to build a startup.

We’re all familiar with Silicon Valley’s mythological image of the tech founder: brilliant, nerdy, eccentric, well-meaning. What you don’t know is that, more than just about anyone else in tech, Wennmachers is the person responsible for harnessing that prototype to build the legend of Silicon Valley. Before Andreessen Horowitz launched in the summer of 2009, most venture capital firms believed that no press was good press. They remained lean, behind-the-scenes outfits and won deals because of their back-room reputations. Wennmachers helped put the firm on the map by pushing its founders, Andreessen and Ben Horowitz, to embrace the press and by helping the companies in their portfolio articulate their ideas publicly. In the years that followed, many firms emulated Andreessen Horowitz’s strategy, hiring marketing and communications leads. As a journalist, I’d often get the call: “Hey, we’re trying to hire a Margit. Do you know anyone?”

Yet it’s the nature of the communications role that we rarely hear much about the people who hold it: The best communicators, by definition, go unnoticed. They’re the invisible third person in every interview. It was Wennmachers who coaxed a reticent Andreessen into participating in Friend’s story because she believed it would be good for the firm. It was Wennmachers who set up a number of Friend’s interviews at Andreessen Horowitz and had a colleague staff them. When Friend felt he needed to see more of Andreessen, Wennmachers hit upon the idea of a TV-watching dinner date, correctly suspecting the scene would be just weird enough to guarantee inclusion and that Andreessen would come off exactly as she hoped: a relatable visionary who identifies with oddball hackers and who, when he is not predicting the future of computers, is watching TV shows about people who predict the future of computers.