She is a friend of Hannah McKay and the sole witness to a murder Hannah committed while the were in a halfway house together.
She and Hannah spent time together at the same halfway house and grew close. One day, she witnessed Hannah feeding rat poison to their counselor, effectively killing him. Arlene once told this to Hannah's father, Clint McKay, who planned on using this information to get rich in the future.
When Clint McKay is unable to extort money from Hannah McKay or Dexter Morgan, he gives Arlene's name to the police. Lieutenant Debra Morgan pays a visit to Arlene and questions her about the murder, but Arlene is reluctant to give her any information about it. Debra then threatens Arlene that her kids will be taken away, which forces Arlene to consider the offer. She tells Hannah and about the offer and expresses her worry. Hannah reassures her, saying that she will take care of the matter. Later, when Hannah is arrested for Sal Price's murder, she and Hannah form a plan for Arlene to help her escape. At the courthouse, Arlene distracts the guards by crying and hugging Hannah, during which she sneaks something to Hannah. This substance causes Hannah to suffer a seizure during her transport and she is taken to a hospital. When Hannah awakes in the hospital, she is able to escape custody. Hannah is last seen placing a plant in front of Dexter's door, after which she disappears
Hannah has been sending some of her money to Arlene to keep it safe. Hannah plans to take it and leave Miami to start a new life. However, Deputy Marshall Max Clayton, is looking for her after being contacted by Jacob Elway. Dexter helps Hannah retrieve her money by driving her to Arlene's house. In return for Arlene's help, Hannah has been financially supporting Arlene and her chlldren, and they now live in a much better house. While there, Dexter spots Clayton outside walking to the front door. He instructs Hannah to hide and instructs Arlene to act like everything is normal. Clayton is surprised to see Dexter at Arlene's place and wonders where Arlene got the money to pay for the house she is living in. Dexter pretends that he is helping Arlene after they bonded over Hannah when the latter went to prison. Clayton leaves, despite suspecting them of hiding something.