But Leone is a man of famous determination. With his Marine drill sergeant demeanour and severe haircut, you know he is a man who will not easily give up.
That, indeed, has been the story of his life. Mr Leone - Gerry - was born in north Boston, on the wrong side of society's tracks, but made it into this city's greatest pride, Harvard University. He managed it through an sporting scholarship, and paid his own way through law school.
In his closing argument we discovered his determination. For a solid hour, he spoke to the jury without notes. He blithely disobeyed Judge Hiller Zobel's instruction to stay within arm's reach of the lectern, pacing intensely back and forth along the jury box. And he saved his best for the last moments of that closing speech. For the first 50 minutes he seemed the Leone we knew, the prosecutor who had barely been able to lay a glove on the defendant in his cross-examination, and had not even tried to press her on the charge itself: had she killed this boy?
But in the last 10 minutes he let rip. In vivid language he described to jurors his picture of what happened on 4 February: a frustrated, furious nanny, grappling with a baby who would not be consoled - a "wet, cranky, slippery, fussy" Matty Eappen, who would not stop crying. Then, he rushed on, she snapped. She shook him and slammed him about the head. "And then Matty wasn't crying any more".
He had saved for these minutes, too, an observation that perhaps said more to the jury than all the medical evidence. Louise said on the telephone to emergency services that she thought Matty had choked on his own vomit. Repeatedly she was told to turn him on to his stomach, but she never did that. Why not, he asked? Because she knew that choking on vomit had nothing to do with his distress.
What sympathy does Mr Leone have for Louise? Scant. "She had numerous opportunities to tell the Eappens and everyone what happened, and she didn't do that."