He was voiced by David Wills.
Don Thousand was once a denizen of the Astral World that got cast away.
when that world decided to throw away the power of Chaos - which Thousand embodies - to be able to rank-up further. The castaway elements formed with time the Barian World, which Thousand ruled, plotting vengefully how to take down his old home world and filling it with Chaos. He clashed with the envoy of the Astral world, Astral himself, eons ago, in a battle that raged on and caused terrible destruction to even Planet Earth. One last move from Astral ended their battle and obliterated the two of them into energy forms, which were able to witness the Numeron Code, the source of all the universe's existence. The explosion also shattered part of Astral's memories, turning them into 50 Number cards that fell on Earth, and enabled Thousand to infect the envoy with a shard of his own malice, which will later become Number 96 Dark Mist. Astral was able afterwards to seal the deity in the Sea of Ill Intent of the Barian world, and 7 of the Number cards, known as Legendary - Mythirian Numbers, became seals for Thousand's true power. To restore his lost potential, Thousand used what remained of his energy to visit Earth in a reaper-like form or posing as a shaman, corrupting the Hearts of seven noble humans, which he orchestrated to become the Seven Barian Emperors, with Hatred and Chaos, to serve later on as expandable tools for his revival, many centuries later, while also planning on gaining the ability to tap into the limitless power of the elusive Numeron code to carry on his plans successfully.
Thousand first anticipated Vector to come to him in the Sea of Ill Intent, desperate to take revenge on Astral and Yuma. He offered him a deal, with Vector becoming his current host and doing his bidding, while receiving great power in return. He made it very clear for Vector that he was the one giving the orders, being able to stop his heart if the Barian Emperor disobeys, though he did so not out of sadism, but to carry on his plans smoothly. Vector used his new powers to accelerate the healing process of Girag and Alito, while also embedding them with much more sinister personalities, courtesy of Thousand, but was left in the dark over many things only Thousand knew about, including on the true purpose of the Mythirian Numbers, which the Deity told him they need to gather, letting him convince the other emperors to help them with this task. Unbeknownst to everyone else, the simple act of removing said Numbers from their location would restore Thousand's powers, something the protagonists achieved oblivious of the true consequences this would have. The Barian god also rescued Mr. Heartland and converted him into his servant, gifting him with a Fake Number to destroy Yuma Tsukumo. He was also aware of Vector's personal agenda, and seemingly cooperated with him, while also building Dimension-fusing machines without the other emperors being aware.
The machines were able to fuse Barian World and Earth into one plane, allowing all the Barians to conduct their assault at full power. Thousand assisted Vector in his personal thirst for power, by brainwashing and then eliminating the other Emperors, absorbing their energy and powers. The heroes discovered that the Emperors, who were once great human heroes and sages, retained false memories of what led to their demise, filling the hearts with hatred and resentment, something Don Thousand manipulated from the start to cause them to ascend to Barian World instead of Astral World (which was filled with "pure and noble" humans), to claim the tremendous power of these beings as his own.
Vector turns on Don Thousand after realizing what Thousand did exactly to his memories, and feeling he has got enough power to slay the Barian god, which he apparently does much to the shock of the protagonists and his nemesis Nash. Unfortunately for him, Thousand was merely playing dead, rose from the ground and proceeded to absorb him with the power of the deceased emperors flowing through him. He emerged in a glowing humanoid form to face Nash and Yuma, until Mizar stepped in to duel him and defeat him, trying to end the duel in a draw at the cost of his life to finish off Thousand in an OTK, which failed and took only him out, due to Thousand mysteriously manipulating something in the duel. Yuma and Nash challenged Thousand to a duel with everything on the line, the deity revealing that he'll cause Astral World itself to take heavy blows each time the heroes get damage as well. Astral deduces that Thousand was using a field spell called Numeron Network, that allowed him to alter the duel's reality in his own favor, which is how he won against Mizar without seemingly doing anything.
Thousand reveals he knew only that the Numeron Code was hidden somewhere on Earth, so by merging Earth and Barian World, he had full control over the reality warping artifact wherever it may be hidden, utilizing its power to fuel his Field Spell. Astral tried locating the card's actual location and relocate it on the duel field, so his comrades may have a chance at disabling it, but Don Thousand anticipated this eventuality, telling the emissary that removing this card will cause all the built-up wild energy in its chamber to be unleashed and annihilate Earth, making Astral willingly try to sacrifice himself by taking in all that destructive energy instead. However, Eliphas, the Astral world's Will, took the blow to save him. Unfortunately, Thousand was far from helpless without his field spell, revealing godlike card after another and pushing the heroes in a corner repeatedly. After an exhausting battle, Yuma was able to finish him off, using cards that symbolize his bond with Nasch and Kite Tenjo. Although unable to believe that a god could lose, Thousand sneered, telling the last surviving emperor that the power of Chaos is eternal and that he will realize the true nature of the Curse the Barian god had cast: Nash's very existence as Barian. (This curse is about Nash willingly choosing to become a Barian to rejoin his fallen comrades, meaning he will defend that world no matter what, regardless of brainwashing).