Bangladesh, Aug. 8 -- In the fast-moving world of cryptocurrency, few companies have grown as rapidly-or as quietly-as Tether. Once an obscure startup behind an experimental digital token, the company has evolved into one of the worlds most influential private financial institutions. Its flagship stablecoin, USDT, now underpins a significant share of global cryptocurrency trading and has become a cornerstone of the digital asset economy. Yet despite its enormous financial influence and estimated valuation of around $200 billion, Tether remains one of the least transparent companies of its size.

The companys extraordinary rise illustrates both the promise and the risks of the cryptocurrency revolution. While supporters credit Tether with ...