‘‘We call 'em "Sixers" 'cause that's the company rule. No name, just numbers.’’
~ – Wade narrating the Sixers they have no name, just numbers.
The Sixers are supporting antagonists in the 2011 book called Ready Player One and its 2018 film adaptation directed by Stephen Spielberg. The reason why they're called Sixers is that each employee has both an employee number and avatar name that is about six digits long. They're contract-bound OASIS soldiers to IOI, which provides them with bi-monthly pay, food, lodging, healthcare benefits, a retirement plan, high-end armor, vehicles, weapons, and coverage on OASIS teleportation fees. In reality, they are people who are in debt to IOI, and some willing employees.
In the film, the Sixers made their first appearance was during a race for the Copper key. They later they ambushed Parzival and Art3mis at the online dance club while they were looking for the Jade Key. Then they were trying to survive the Shining movie in order to get the Jade key, dying many times over. Later, F’Nale Zandor and a group of Sixers find the Resistance. Wade escapes, but Art3mis amongst other people are taken to a loyalty center to pay off debt. Lastly, the Sixers seen on Planet Doom in the climax of the film. The Sixers are laying explosive charges before Art3mis breaks free of her rig and takes down the force field IOI had put up earlier. Wade convinces nearly every single OASIS player to join them and stop IOI on video, and the players rush into the third challenge, taking out many Sixers in the process. Countless Sixers are taken out in large numbers when a Chucky is released, when Wade uses his Holy hand grenade, and when Nolan Sorrento's avatar dies and his MechaGodzilla mech falls on top of the Sixers. After the last Sixer in the castle dies after winning Adventure, Wade deduces that the way to win is by finding an easter egg in the game. To stop Wade from winning, Sorrento set off the Cataclyst, a bomb that killed everyone on Planet Doom, including Sorrento and the Sixers. At the end of the film, Wade says that all loyalty centers had been then on refused access to the OASIS, and had to be shut down, freeing the Sixers from their indentured servitude.
In reality they're a bunch of men and women wearing virtual reality helmets and use these devices that allows them move throughout a massive-multiplayer virtual reality simulation called the OASIS. However in the OASIS they have a helmet with the name of the company, and a chestplate that has a 6-digit number.
Innovative Online Industries enslaving people to become Sixers would probably have been prohibited by law enforcement, but Sorrento was only arrested at the end of the film.
An ad for the Sixers seen in the film suggests that some of the Sixers were willing to fight.
What backs this up is that it happens in the book.