Tim Leissner, the former Goldman partner pleaded guilty in the summer of 2018 and has been cooperating with the government. Mr. Leissner, who is married to the fashion designer and model Kimora Lee Simmons, agreed to forfeit up to $43.7 million as part of his guilty plea. One of Goldman’s most powerful deal makers in Asia before the scandal erupted in 2016, Mr. Leissner is expected to appear on the stand for several days once he begins testifying. Given his ties to Roger Ng and Jho Low, his testimony is seen as crucial to the government’s case.