McNamee has been an unseen presence in the marble-walled courtroom for the past month, but most information about him has been revealed in snatches of testimony, some of it contradictory, from four government witnesses.
When Clemens' former trainer takes the stand, as he is expected to do Monday, jurors will see a practiced witness with a checkered past and inconsistent testimony whom Major League Baseball, members of Congress, the FBI and prosecutors nonetheless believe.
The jury will get a chance to measure the man who the government says injected Clemens with steroids and human growth hormone, but who the defense says is a gold-digging liar who manufactured needles and swabs with steroids and Clemens' DNA.
So far, what the 12 jurors and three alternates know about McNamee, who lives in Long Beach, has been carefully shaped through legal skirmishes between Assistant U.S. Attorney Steven Durham and Clemens lawyer Rusty Hardin.
The result has been a patchwork of testimony.
"I had a good relationship with him. He did a great job training me," Pettitte told the jury, saying that he still considers McNamee a friend.
But what Pettitte could not testify to under the judge's ruling, and what the jury does not know, is that McNamee injected Pettitte with HGH in 2002.
Federal agent Jeff Novitzky told the jury McNamee was an "extremely nervous" government witness who often complained of health problems and other commitments. McNamee cooperated with Novitzky's investigation of steroids in pro sports to avoid possible distribution charges, the agent said.
Another witness, confessed drug dealer Kirk Radomski of Manorville, told jurors he sold steroids and HGH to McNamee, a former New York City police officer. He also called McNamee a friend.
General manager Brian Cashman testified that in 1999 he hired McNamee to train only Clemens after the pitcher requested his help. Yet other Yankees "gravitated" to McNamee and used him, too, he said.
Cashman said he let McNamee go two years later after coaches and staff complained about him and following off-field incidents in 2001 in St. Petersburg, Fla., and in Seattle.
But after bench conferences and a huddle in the hallway with prosecutors, Cashman could not reveal what had actually happened in those two cities: a sexual assault investigation that resulted in no charges and McNamee's passing out drunk in the team's hotel bar.
Monday, U.S. District Judge Reggie Walton will hear arguments -- out of the earshot of jurors -- about a defense motion to introduce into court McNamee's contentious divorce.
Hardin wants to expose jurors to charges by McNamee's ex-wife's that he entered her house without her permission, had a substance abuse problem at the time he said he injected Clemens, and was involved in a prescription drug ring.
Durham has fought to keep the divorce file sealed.