Six is a spectre, and was a part of Emperor Arcturus Mengsk's attempts to revive Project Shadowblade. He often smiled, but never with a friendly expression. Though given no real name, his designation was one given by terran reporter Donny Vermillion in order to tell the spectres apart.
Five was part of an attempt to revive the failed Project Shadowblade on Tartarus. Overseen by Evelyn Yoo, these spectres would be fiercely loyal to Emperor Arcturus Mengsk, and was to be used as a weapon against the zerg, but they were stored in cryosleep. However, after Arcturus Mengsk’s death in the Second Great War, Tartarus fell into obscurity, though the staff continued their research. This was disrupted when the zerg they tested on broke out and began to overrun their base.
Six and his spectre allies were released by Reigel, who along with Nova Terra and Donny Vermillion, came to investigate the jorium stockpile on Tartarus but found the hidden spectre labs. Six and his allies initially followed Reigel out, but soon revealed they intended to continue following the orders of Arcturus Mengsk, and knocked him unconscious. They arrived on the roof and met with Donny Vermillion, who welcomed them as heroes, but the spectres prepared to kill him. Vermillion convinced them he could update them on what has happened since they were cryogenically frozen. They kidnapped Vermillion and stole Nova’s ship, the Griffin, and departed the planet.
The spectres began to strike targets through the Dominion in order to cause chaos in Arcturus’s name, but on the ship, Three and Five began to doubt the wisdom of fighting for a dead emperor, while Four supported continuing their campaign. One affirmed that they were uplifted by the emperor to continue to serve after his death, and that they had to stick to their mission. Six remained silent throughout the argument.
Later in their warpath, the spectres landed on a remote mining planet, demanding from the town that they get vespene gas to continue their revolution. One, Two, Three, and Six confronted the mayor directly. Though the mayor was a support of Arcturus, he did not want to get involved with a rebellion. Just as One prepared to attack him, Daelaam forces in a void ray arrived, allied with Nova Terra. One yelled at Vermillion for selling them out, to which Donny admitted to broadcasting a report on them. As they closed on him, the infested terran Jun-Ho Yoo attacked, and Six stabbed him in the chest, killing the infested child. Knowing there was no time to deal with Vermillion, One let him live and prepared to face Nova.
Six and his allies then confronted Nova and her dark templar escort, armed with monomolecular blades. The spectres fought against the Nerazim and Nova in a fierce melee, and one of the six spectres were killed, but as they began to get the upper hand, Nova pointed out the void ray above them would wipe them out at her word. She and Reigel then discussed with them how they didn’t need to die for a dead emperor, and they could choose their own path going forward. A spectre threw down their blade, enraging One who lunged at them, but he was restrained by his allies, who all decided to surrender to Nova.
The spectres were taken to Aiur by Matriarch Vorazun, and together they worked on stabilizing the modifications the terrazine had done to them, physically and psychologically. After a time of training in a temple on the remote regions of the planet, Reigel returned with the Griffin, and Nova and the spectres approached it. 
Six is possibly the spectre killed during the confrontation with Nova, though it could alternately be Four. The details of the spectres as a whole are listed here for the sake of completedness.
- Brian Alexander, Akex Acks (w), Carlos Rodríguez (i) and Sandra Molina (c). (April 2, 2022). StarCraft II: Nature of the Beast - The Complete Collection. Blizzard Entertainment. ISBN 978-1950366866