Parker first appeared, when he came to Oliver's cell after he was sent to Level 2 for his therapy. But, because Oliver was unwilling to answer his questions, He commanded the guards to inject him with truth serum to make him answers his questions without resisting. After several session, he tried to ask him his reason to become Green Arrow. Oliver told him that he and his father along with a man named Hackett were deserted at the ocean in a life raft. And, his father killed Hackett and himself to ensure his survival and made a promise with his father to stay alive and right his wrongs.
Then Jarret convincing him that his father was a murderer like any other murderer and his promise with his father burdening him with his sins that he doesn't have to carry, and his vigilantism would become a bad influence for his son, William. After he refused to let go of his past, Jarret puts him in a machine that directly interacts with parts of his brain containing memories and then manipulate and stimulate them as mental simulation. When Oliver is put on the same situation as he was in the past with him being his father and William being his younger self in a simulation, he did exactly the same thing as his father did and by showing this, he told him to end the cycle his father started for his own son.
As a psychiatrist, Parker is a calm man who would never let his emotion get the best of him. He would always try to use a peaceful approach when handling a patient. But, he would resort to forceful and violent approach if the patient is not willing to cooperate. When handling Oliver, he was seems to be very cunning and manipulative, always striking people's vulnerable sides. He is heavily against vigilantism, as he was really desperate and determined to make Oliver give up his Green Arrow identity through many methods.