Who is Stephen from Faithful Place: A Novel and what is their importance?

Stephen is a young police officer who is working his way up. He is selected by Frank as someone who will feed him information from the case. Stephen is smart and doesn't initially believe that filling Frank in on details is ethical. He eventually comes to agree with Frank's assessment of the case and works with him. When Frank calls Stephen off the case, Stephen is furious. Stephen steps up when Frank arrests Shay and takes Shay into custody, taking the lead on the murder case.