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The depicted man is clearly a Westener, and the costume is indeed a Korean Choson period costume (ch'ônik). The drawing is part of a series of five drawings -- the other four being men in Chinese costumes. It has been speculated that the man in the costume is the China missionary Nicolas Trigault (alias Kin Nige, 1577-1628), who in January 1617 was together with Rubens in Antwerp.
Courtesy Frank Hoffman and Henny Savenije.