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As of yet it is not yet certain who the suit-wearing man is, although it is commonly agreed that he has a pivotal role to play in the upcoming Half-Life 2: Episode 3. Avid followers of the series will know that he appears in almost every game level (see G-Man Locations), and lots of fan sites have been set up in his honor.
Many speculate that the G-Man is actually Gordon Freeman, the star of the games and who you play as.