Dianne McKeever doesn’t like to talk about the fact that she’s a woman. But her gender has made her a groundbreaker on Wall Street: The moment she launched Ides Capital last year, the 38-year-old became the only woman running an activist hedge fund shaking up U.S. companies. And her firm, which takes its name from the day of each month on the Roman calendar thought to portend dramatic change (made famous by Julius Caesar’s assassination on the Ides of March), is already showing companies that they ignore her at their peril.

Shares of Boingo Wireless wifi , a provider of Wi-Fi hotspots (you’ve likely used it at airports) have risen more than 20% since McKeever revealed her campaign to amplify the company’s value in March. For Ides, the rewards have been even greater: Its ­investment has returned more than 55% since the firm began buying the stock in late 2015.

McKeever has made corporate governance a pillar of her strategy, and in doing so she has become known as a champion of boardroom gender diversity. After Boingo’s stock price lost nearly half its value since going public in 2011, McKeever sought to replace two members of the company’s board, which she criticized as “stale, pale, and male.” (On the cover of her presentation about the company, she quoted Cicero: “The Ides changed everything.”)

Dianne McKeever is number 19 on our 2016 40 Under 40 list. See the full list here.

Pushing to right society’s wrongs is not Ides’ goal. “We’re there first and last to make money,” McKeever says. Still, Ides’ efforts set it apart from its peers. In the past five years the top five activist hedge funds nominated women for just 4% of the board seats they targeted, according to Bloomberg. That’s despite evidence that gender diversity correlates with financial performance: Companies in the MSCI World index with at least three women on their board posted an annual return on equity of 10.1% over a six-year period through last summer, compared with 7.4% for companies without.

McKeever’s initial challenge was getting others to take her seriously. When she approached Boingo with her new-director nominees, the company said it “had never heard of Ides Capital.” McKeever and her cofounder, Rob Longnecker, owned just 0.18% of Boingo’s stock at the time, far less than the stakes of 5% or more that activist investors often buy to gain leverage over a company. “Normally, a holder of that size you don’t really need to listen to or negotiate with; her chances of winning were exceedingly low,” says Karen Finerman, CEO of Metropolitan Capital Advisors and one of ­McKeever’s nominees to ­Boingo’s board. “And she did.”

In June, Boingo settled. Ides had gained the upper hand when Institutional Shareholder Services, a proxy advisory firm that holds enormous sway in board contests, sided with ­McKeever and recom­mended that investors vote in favor of her slate. Boingo agreed to appoint three new independent ­directors—including its first woman—though not Ides’ nominees. “For a little group to win a somewhat contentious nomination process was huge for us,” says Brad Stewart, CEO of XOJet and Ides’ second nominee. (Boingo says it “had a very positive growth story before Ides became involved” and that it was already trying to recruit women to its board.)

While relatively unknown on Wall Street, McKeever is no rookie. Growing up in Indianapolis, she bought her first stock at age 10 and soon started her first business, arbitraging candy bars: As a seventh-grader taking high school classes, she would buy $20 worth of premium chocolate at the high school, then sell it at a markup to her middle school classmates. “It was what the market could bear,” she says.

After graduating from college in 2001 (with dual degrees in chemistry and chemical engineering at New York University and the Stevens Institute of Technology), she got a job at New York hedge fund Barington Capital Group. By age 25, she had become the youngest person to make partner. “She’s analytical and very smart,” recalls James Mitarotonda, Barington’s chairman and CEO.

At the time, Barington was becoming a pioneer in the current generation of investor activism, and it was there that McKeever got to know Starboard Value activist Jeff Smith, whose hedge fund was then part of Ramius Capital. Smith partnered with McKeever in many of Barington’s campaigns.

But McKeever yearned to make a bigger impact. So she went to law school—not to be a lawyer, but to study up on corporate governance. She launched Ides in the summer of 2015, renting a desk at a swanky coworking space a block from Central Park. She’s now looking for new office space, and she has set her sights on several more small-company targets.

In the meantime, ­McKeever, who describes herself as a “hard-core videogamer,” blows off steam playing Ms. Pac-Man. Her Twitter picture is the game’s avatar; her bio just says, “Eatin’ ghosts.”

Just as in Ms. Pac-Man, McKeever is likely to face greater challenges as she ­levels up with new campaigns. But she’s equal to the task. “The next time that some board sees her filing,” says Finerman, “they do need to worry about it.”

A version of this article appears in the October 1, 2016 issue of Fortune with the headline “Meet Activism’s New Face.”