It took just a few months for a 36-year-old entrepreneur to take over America's most talked-about company — and make $1bn for himself in the process.

Ryan Cohen rose to become chairman of GameStop with the verve of an old-school corporate raider. Wielding little more than a minority stake and a sharp tongue, Cohen pushed out GameStop's executive team and installed longtime associates on the company's board. The tactics made the co-founder of online pet store Chewy a favourite of the individual investors who sent GameStop's stock on a roller coaster this year; they call him "Papa Cohen."