Who is Dr. Arthur Lenz from Mortal Fear and what is their importance?

Lenz is the FBI psychiatrist who talks with Harper during his first interview. At the conclusion of the interview, Lenz asks Harper what instrument he plays, then says that he knew he played because of the calluses on Harper's hand. Lenz eventually pretends to be "Lilith" online and lures Berkmann into several conversations. Though Lenz believes he's getting close, Berkmann has really discovered the truth and has Kali kill Lenz's wife in retaliation.