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Thornton Duggan is the nephew of infamous industrialist Avery Duggan, head of the Duggan Crime Family. He is less dangerous than him, and betrays him to buy the Diamond Casino, becoming the new owner.
Despite being less dangerous and saving the casino, he is a sociopathic and cocky individual, going as far as killing his own uncle and celebrating it after, and firing Vincent, showing no remorse for doing so.
Events of GTA Online
The Diamond Casino & Resort
Thornton is first encountered during the mission Casino - Strong Arm Tactics, when the protagonist/s, Tao Cheng, his Translator, and Agatha Baker have a meeting with Avery and Thornton.
The meeting goes sour, as Tao pulls a gun and threatens Avery following a racial remark, and Avery, seeing they will never sell the casino, orders his men to eliminate them.
Thornton later appears in the mission Casino - Bad Beat to tell everyone he still wants the casino, and that he and his uncle had a falling out. In an attempt to gain their trust and butter them up, he warns them of an incoming attack of The Professionals, hired by Avery to destroy the casino.
In the final mission, Casino - Cashing Out, as Tao Cheng and his Translator are about to leave for China due to his uncle telling them to immediately sell the casino, Thornton returns and informs them of a plan to save the casino, but mentions he won't have the money to do so unless they kill his uncle, thus giving Thornton his inheritance.
Avery Duggan is killed and Thornton celebrates his death with the rest of the casino crew but not before dismissing Vincent, and later becoming the new owner.
The Diamond Casino Heist
In December 2019, Cheng Family Holdings vice president Georgina Cheng arrives in Los Santos and utilizes her hacking skills to arrange a meeting between Lester Crest and the GTA Online Protagonist in Mirror Park. As soon as Lester realizes that he's been set up, Georgina appears and introduces herself. She requests that Lester come out of his retirement and work alongside the protagonist to help her plan an elaborate heist on the Diamond Casino & Resort to take revenge on Thornton Duggan.
After careful planning, the casino is robbed multiple times, humiliating Thornton and costing him millions of dollars due to his insurance premiums.
- Thornton share similarities with Donald Love from the 3D Universe:
- He also shares similarities with Thomas Stubbs from The Lost and Damned, as both of them are businessmen who task the protagonist (Johnny Klebitz for Thomas and the GTA Online Protagonist for Thornton) with killing their uncles, though Thornton became a hidden antagonist whereas Thomas didn't.
- He is one of the two antagonists in the series who weren't killed by anyone. This makes Thornton one of the two antagonists who got away with their actions, alongside King Courtney from Grand Theft Auto Advance.