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SoftBank said in a statement Friday that Tadashi Yanai, the billionaire founder of Fast Retailing, would step down from its board at the end of the year.

Why it matters: Yanai has been on the company's board for 18 years, and is reported to be one of SoftBank CEO Masayoshi Son's closest advisers.

Yanai was also able to question some of the investment decisions made by Son, who has come under fire in recent months after his huge bet on WeWork went south.

During SoftBank's most recent earnings call in November, Son said that Yanai was a "challenging guy" who "always gives me a hard time."

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