Mr. Won is revealed as the driving force behind the mysterious apparition that appears in "Green Ghost" after Bob, Pete, and Chang are captured by Jensen in the mines of Verdant Valley. After their capture, the boys are taken to San Francisco to the home of a rich man, who is shortly revealed to be Mr. Won, an elderly Oriental man. Mr. Won is especially interested in acquiring the string of Ghost Pearls that was found in the coffin in the Green mansion in Rocky Beach because they are supposed to extend life when ingested after being dissolved in a liquid. To that end, he bought the mortgages on Lydia Green's property and manipulated Harold Carlson to try to obtain the pearls prior to the appearance of the Green Ghost.
After the boys discover the pearls and hide them in the mine, Mr. Won is anxious to get them to reveal the hiding place. He has a commanding presence and is used to getting what he wants. Mr. Won's last recorded words in "Green Ghost" are one of the most icy statements in the entire Three Investigators series as he tells Jensen what to do if the boys double-cross him--"you may slit their throats."