Molly's Game: Who Was Mr. X in Molly Bloom's Poker Games?

In the movie Molly's Game, there is a character called Mr. X, a celebrity who plays poker in Molly Bloom's games. Who was Mr. X? Which celebrities played?

Molly's Game - Metacritic

Jessica Chastain leads charge to change Hollywood, onscreen and off – East Bay Times

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Molly's Game - Rotten Tomatoes

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