Killer Joe

In West Dallas, Texas, 22-year-old drug dealer Chris Smith, desperate to pay his debt to loan sharks, decides to murder his mother, Adele, to collect the $50,000 of insurance money. He has been told by his mother's boyfriend Rex that the sole beneficiary will be Chris' younger sister Dottie. Assuming Dottie would share any money she gets with them, Chris manages to rope his dim-witted father Ansel into a conspiracy to kill Adele â who is Ansel's ex-wife â to get the money. Chris also enlists the help of Joe Cooper, a police detective who has a side career as a contract killer. Ansel plans to split the money four ways between themselves, Dottie, and Ansel's new wife Sharla. Dottie hears the plan as they are talking, and agrees that it's a good idea.
The plan almost fails when Chris is unable to front Joe's fee. However, Joe is interested in the odd, childlike Dottie and offers to take her as a "retainer" until the insurance comes through. Through Dottie's interaction with Joe, it is revealed that Adele tried to kill Dottie once when she was an infant. Joe "dates" Dottie and then appears to be staying over at their home and having sex with her regularly. Chris has a change of heart and asks him to call off the hit, only to discover that Joe has already killed Adele. With Chris' reluctant help, Joe hides the body in a car and torches it.
After Adele's death is discovered, the family learns that the insurance claim actually pays to Rex, rather than Dottie. Chris admits he originally heard the details about the policy from Rex, who also told him about Joe. The family all realize that Chris was duped into hiring someone to kill Adele. Immediately afterwards, Chris tries to talk Dottie into running away with him to escape the loan sharks. Dottie says she will go with him, but she must see Joe again first.
After Ansel and Sharla return home from Adele's funeral, they find Joe inside with Dottie. He comes out of her room and asks increasingly pointed questions of Sharla, which ultimately leads her to admit that she knew the policy was really $100,000 (accidental death is double). Joe shows them a check of that amount payable to Rex, as well as incriminating photos which prove Sharla was having an affair with him. Angered, Ansel declines to protect Sharla when Joe punches her and forces her to simulate oral sex on a piece of fried chicken.
Joe knows Chris is coming to take Dottie away and he threatens to murder Ansel and Sharla if they don't stop him. After Chris is seated for dinner, Joe announces that he and Dottie will be married. Chris refuses to let them, ordering Dottie to leave with him; Joe tells her to stay where she is. For a moment Dottie sits there, then she gets up and turns and, while the men yell out at her, Chris threatens Joe with a gun and the two struggle. Ansel and Sharla jump in to assist Joe as he brutally beats Chris. In all the confusion, Dottie recovers the gun and, in a rage, she fires several shots, killing Chris and seriously wounding Ansel. Dottie turns the gun on Joe, telling him that she is pregnant. Joe appears overjoyed as he inches closer to Dottie. The film ends just as Dottie moves her finger back on the trigger.

Question: Joe starts a physical relationship with which character?
Answer: Dottie