Andrew Klein is a manager formerly associated with the Miller Gold Agency who made his debut appearance towards the end of Season 5, in the episode Pie, as a successful partner at the Klein-Cutler Talent Agency, a stable of TV writers from the Valley.
In Season 6, it was revealed that Andrew had become the head of the Miller-Gold Agency's television department and his friendship with Ari Gold continued to go from strength to strength but things took a nosedive when in the episode, Amongst Friends, it was discovered that Andrew was having an affair with junior agent Lizzie Grant.
The next morning, Ari arrived at work, angrily confronting Andrew over the affair with Andrew swearing on the lives of his children that he had ended the affair with Lizzie but a few hours later, after Andrew and Ari had spoken to each other in a garage, Andrew later rang Ari, telling him that he had told Marlo about the whole affair and that he had left her much to Ari's surprise.
In Murphy's Lie, Andrew's wife, Marlo arrived at the office, demanding to see the woman who was sleeping with her husband and her actions later prompted Ari to fire both Andrew and Lizzie although Lizzie protested against it. However, Lizzie was spared from being fired although Ari vowed that if she ever had another affair with another agent, he would deport her naked to the Taliban.
Despite Andrew's reluctance, Ari advised him to go back home to Marlo who Ari believed would take Andrew back with Andrew agreeing to do it.
However, in The Sorkin Notes, it was revealed that Marlo had refused to take Andrew back, telling him that he couldn't live with her anymore and as such, Andrew had resorted to sleeping on Ari's couch much to the amusement of his colleagues and Ari's own horror.
But despite this setback, Andrew insisted that he would be able to sign Aaron Sorkin and later left the office with Barbara Miller spotting at him and upon learning that it was Andrew, she stormed off to the office to confront Ari, telling him that if he didn't fire Andrew, she would.
Ari began panicking when it was discovered that Andrew hadn't shown up for a pre-meeting due to the fact that Andrew had gone back to his house in hopes of getting some notes he had been working on for his talk with Aaron Sorkin.
Unfortunately, Marlo refused to let him in and after a stand-off, Andrew opted to head back to the agency, only to discover that Marlo had his notes and when she set them on fire, Andrew snapped and drove his car into his house, presumably intending to kill Marlo but due to her fleeing in time, Andrew ended up crashing into his living room.
Ari's assistant, Lloyd Lee later arrived into the meeting and informed Lizzie, Sorkin, Barbara and Ari that Andrew was in jail much to Ari's dismay. Andrew being in jail later prompted Ari and Sorkin to personally visit him in jail
In Security Briefs, Andrew arrived in, revealing that Sorkin had decided to sign with the agency.
However, in Bottoms Up, it was revealed that Klein had gone into rehab for sex addiction and that he was also let go for hiring prostitutes using company funds with Ari even stating that Klein would never work with the agency ever again.
Klein later called Ari while in rehab to warn him Ari about Lizzie's maneuvering.
In the Season 7 finale episode, Lose Yourself, during a talk between Ari and Marlo, it was revealed that Klein was due to get out of rehab the week after the events of the episode.