He was one of the FBI guys, I think his name was Archer.
When Leo was young and needed a heart transplant Archers son was in the same hospital. he had been waiting a couple of years for a transplant as well.
Leos mum, because she had money and power, got leo put to the top of the transplant list.
Subsequently Archers son died, he blamed Leos dad and went on to try and ruin him.
What I've said doesn't really do it justice.
it was a cracking final episode, I recommend you try and see if you can download it from somewhere as there were lots of twists in it.
if you do i hope I haven't ruined it for you - Oooops!