Diane was the love interest of Willie Soke in Bad Santa 2 and she was played by Christina Hendricks.
Before the movie starts, she was married to Regent and was happy with her job at the charity. But her relationship with her husband was difficult since she didn't know that he was cheated on her and thinks that he and her never have sex ever since.
Few years have passed, Willie is annoyed to learn that not only is a target of the con a charity, but that his estranged mother, Sunny, who uses "shit stick" as a term of endearment for Willie, is Marcus's contact.
Willie reluctantly agrees, since Sunny is suffering from early stages of Parkinson's Disease, though he secretly makes a deal with Marcus to cut Sunny out when the time comes.
Forced to don the Santa suit once more, Willie is arrested when he beats up another Santa (after a misunderstanding causes Willie to think the other Santa is a pedophile) but is bailed out by one of the charity's founders, Diane.
Though she wants to fire Willie for his behavior, she relents when he agrees to attend AA meetings with her.
Diane's husband Regent, who also runs the charity and is cheating on Diane with his secretary, orders his head security guard Dorfman to track and follow Willie, becoming suspicious of how much time he's spending with Diane.
Meanwhile, Marcus, after doing recon work, tries to seduce Gina, another security guard so he can obtain the keys to Regent's office where the charity's safe is kept but he fails to do so as Gina is high maintenance.
Elsewhere, while Willie starts a sexual relationship with Diane, he and Sunny start to bond, especially when they pull a job just between the two of them, robbing a mansion with Willie posing as Santa and Sunny as Mrs. Claus.
Bonding over their haul afterward, Sunny gives Willie a gun so he can betray and take revenge on Marcus despite Willie’s insistence that Marcus has changed.
Thurman soon arrives in Chicago, having followed Willie to the city. Willie at first considers leaving him at a laundromat but soon takes him to the shelter at the charity.
Thurman ends up joining the children's choir, which is set to have a concert on the night of Willie, Sunny and Marcus's heist. Willie soon encounters Gina, who, thanks to Sunny's prodding, believes Willie wants to sleep with her.
They end up having sex in the bathroom of a bar, and Willie obtains the keys. On the night of the show/robbery, Willie catches Thurman singing, which makes the kid very happy.
As Willie cracks the safe, Marcus is about to betray him again, but they escape just as Regent and Dorfman discover the con. However, Sunny reveals she is betraying them both, telling Willie the bullets in the gun she gave him are blanks before shooting Marcus.
Then, she drives and attempts to make a get away at a Santa Con Event. Willie, Regent and Dorfman chases her; despite her blending in, Willie catches her and attempts to take the bag of stolen cash by saying the kids need it more.
He then shows her the flier of the Santa Con event proving he was on to her. Sunny threatens to shoot him if he doesn't let go, but she shoots anyway and the shot is muffled by fireworks, shocking Willie.
In the final struggle, Sunny kicks the bag accidentally releasing the money. Enraged, Sunny attempts to shoot him again, but Thurman ends up taking the bullet in his behind while trying to give Willie a hug.
Both Sunny and Willie are arrested, but Willie is not charged due to his help in catching Sunny. While he recovers, he is visited in the hospital by Diane who wakes him up with manual pleasure.
After that, she left and it's a hint that she may be in a relationship with willie. It's unknown if she left her husband or not as the movie ends.