Fahmi Quadir, known as the assassin on Wall Street for her short-selling prowess, is reinventing herself after a decade of betting against companies like Wirecard. Her work uncovering corporate fraud earned her two Netflix documentaries and a fearsome reputation.

Now 35, the American hedge-fund manager is walking away from short-selling to become an activist investor in South Korea's booming market. From her London base, she rises at 2 a.m. to research hidden gems, planning her activist debut this summer.

The shift marks a dramatic turn for Quadir, whose fund Safkhet Capital operated with no hedges or long positions—only bets against alleged frauds. Launched in 2018, the fund initially succeeded but struggled as the S&P 500 nearly tripled, making short positions increasingly costly.

"I kind of felt like a hamster running on a wheel," Quadir said. "No matter how good the ideas were, it was just too difficult to get out of the hole." She now seeks to push stocks up rather than drive them down.