David Webb made his fortune in Hong Kong by weeding out the bad companies from the good on the city’s stock exchange. By the age of 32, he had earned enough to leave his job and decided that he would start ruffling some feathers in his adopted home.

In the three decades since, Mr. Webb, who moved to the city from his native Britain, has almost single-handedly taken on the rich and powerful in the financial hub, tirelessly campaigning for greater transparency from the city’s public companies about their ownership and their financial dealings.

The activist investor tussled with tycoons who dominated Hong Kong’s economy, crossed swords with regulators to fight for the rights of minority shareholders, and lifted the curtain on shadowy business transactions.

His biggest expose was publishing a report that documented previously unknown connections between a network of 50 companies that he called the “Enigma Network.” The report caused their shares to tank and prompted investigations by government watchdogs into share price manipulation by the companies’ owners that hurt small investors. One former regulator calls him the “conscience” of Hong Kong’s financial markets.

“I could go out and criticize behavior and companies and not feel like I would become unemployable,” Mr. Webb said in an interview at his apartment, where floor-to-ceiling windows offered a sweeping view of the city’s financial district.