BOSTON — James Bulger, the second-most notorious man in Boston, sits there in blue jeans and white sneakers, his balding head shaved to a salt-and-pepper speckle, his glasses perched at nose’s tip in faux-studious pose. Somewhat diminished by his 83 years, he looks as if he might be waiting for the discount bus to Foxwoods Casino.

But then testimony resumes at the federal courthouse here on Boston Harbor, and the words transform the would-be retiree called Whitey into a sociopathic thug in winter — one who seems less perturbed by descriptions of him as a serial killer than by the recurring contention that he was a longtime government informant. He has his pride, after all.

Perhaps the most damning of those words came late this week from a hearing-impaired witness of 79, wearing a green windbreaker and a quizzical look that suggested he too was looking for the Foxwoods bus. Only this was Mr. Bulger’s former partner, Stephen Flemmi, a stone killer now serving a life sentence in prison for assorted crimes against humanity.

In less than 15 minutes of testimony on Thursday, Mr. Flemmi managed to acknowledge his role in 10 murders (Richie Castucci? “Yes.” Roger Wheeler? “Yes.” Debra Davis? “Yes.”), admit to his own moonlighting as a veteran informant (a “quid pro quo” arrangement, he said), and describe Mr. Bulger as a federal informant whose diligence might have qualified him for a government pension.