Jeff Doyle is District Attorney and is investigating who shot Meredith Kane (assuming the shooter was aiming for Tom).
Meredith reveals that he has a tense distant history with Tom which likely refers to Jeff's annoyance that had his suspect's identity (Mr. Langley) not been leaked that the true shooter accomplice would now be in custody. He is professional though his office has been treated with disdain.
In Season 2 Episode 5 at the height of Tom Kane's symptoms, Kane phones Jeff Doyle to report corrupt officials to arrest. A month later, Jeff informs Kane that he will have wide ranging subpeonas back to 1993 about that corruption to make airtight convictions.
Doyle is confident that he doesn't need to be in alliance with the mayor's office (suggesting he has powerful influences somewhere else).
In Season 2 Consequences, he has asked an employee, Claire, to get close to Zajac. He directs her never mention his wife's name, whom he says is a saint. Then he tells her if she wants to be his successor, she must do as he says, and proceeds to ask her to sexually satisfy herself in his office while he watched.
The actor's name is John Hoogenakker