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Kroton was a Cyberman who uncharacteristically retained his emotional capacity.
He discovered this on the planet Mondaran, where he turned against his superiors and saved a group of resistance fighters, "sacrificing" himself in the process by launching the spaceship stolen by them into space while his batteries died. After being re-activated by a group of Humans, Kroton spent many years travelling alone; saving planets, and helping those he came across. Occasionally he would have brief flashes of memories of a lost life come into his mind, which were real enough to reach his mind but not concrete enough to reveal to him more than brief pockets of his past. After helping the Eighth Doctor and Izzy on the Qutrusian Cargo Freighter X-703, Kroton was taken on-board the TARDIS, where he travelled with the pair for a time. After a battle with the Master and Sato Katsura, Kroton regained his memories from his life as a human and became the new centre of the Omniversal Spectrum.
Little was known about Kroton's original life. He had a family and a wife named Shallia, who was also his childhood friend. At some point, he was taken from them by the Cybermen and converted. He himself forgot all of his past, including his real name, and only knew life as a Cyberman. (COMIC: Unnatural Born Killers, The Glorious Dead)
Early life as a Cyberman
On Telos, Kroton became a Junior Cyber-Leader, and was sent to assist in the management of the occupied planet Mondaran. While leaving the planet, he wondered if he would ever see its sky again, a thought which his associate Liron found to be against what his programming should allow.
On Mondaran, Kroton helped in the investigation of the resistance forces. He helped capture the elderly Willoway, and told the Cybermen under his orders not to kill Marilka, who was attempting to stop them from taking Willoway away. The resistance fighters would later find this puzzling, as Cybermen usually killed at the first sign of hostility. Kroton was horrified to learn that Willoway had chosen death over the act of giving information to the Cybermen. He could not understand how Willoway would choose to protect something other than his own life, which he had been thought humans were very precious of.
Kroton attempted to contact members of the resistance movement to question them on this, but they reacted hostilely to his arrival. Despite this, he kept calm and continued to inform them that he just wanted to talk to them until they fled. When a group of Cybermen arrived to look into the disturbances of the evening, Kroton turned them away, telling them to rejoin their troop.
Kroton met with the traitor Zarach, a humanoid who sold secrets of the resistance to the Cybermen in exchange for his life. He questioned why Zarach would betray his own kind, to which Zarach explained that he was like the Cybermen — emotionless.
The information from Zarach was used to arrest the leader of the resistance, Pendar. Kroton went to speak with Pendar, and chose to save him from the prison and take him away from Mondaran City, where he let Pendar go. Kroton then began to sulk quietly, not sure what he had done or why.
Pendar briefly considered killing Kroton with a large boulder, but found that he could not, as it would make him no better than the Cybermen. Reunited with Marilka, the three began discussing the events, where Pendar realised that Kroton had retained some of his human emotions. Kroton was highly doubtful of his place in the universe -- if were to return to his life as a Cyberman, he would be deactivated, but as a Cyberman he would not be trusted by any species but his own. Pendar disagreed, saying that once the resistance had won against the oppressive Cybermen, Kroton would have a place among his people.
The three decided to steal a shuttle as it was an unexpected tactic. They were attacked by Cybermen on the base they chose but were able to get to a small ship. The ship they used to escape was low on fuel, meaning that the three had to stop off in the centre of the wilderness. Kroton's battery packs were running low, and knowing that he had no way to recharge them and that the shuttle would only key the other Cybermen into his friends' position, he chose to use the remaining fuel to shoot out of the planet's atmosphere, saving Marilka and Pendar but leaving him lifeless, floating in space for years. (COMIC: Throwback: The Soul of a Cyberman)
A cruise spaceship, the Flying Dutchman II, which was stuck in a lateral stasis field, found Kroton's ship and the human passengers reactivated him. He helped remove the ship from the stasis loop, but discovered it had been trapped for six hundred and twenty-eight years. The mortal passengers all rapidly aged and died, leaving Kroton alone again, with only the ship's robot pilot for company. (COMIC: Ship of Fools)
Kroton continued to travel through space. Along his journeys, he helped a group of Technosmiths on Baroq VII. They thanked him by making some "special adaptions" to his armour — which allowed Kroton to withstand blaster fire from various guns. (COMIC: The Company of Thieves) Kroton prevented an act of genocide by the Sontarans. (COMIC: Unnatural Born Killers)
A companion of the Eighth Doctor
Kroton encountered the Eighth Doctor and his companion Izzy on the Qutrusian Cargo Freighter X-703. After assisting them, he joined them in the TARDIS. (COMIC: The Company of Thieves) Kroton left the Doctor after a battle with the Master during the Master's pursuit to become the centre of the omniversal spectrum. He reached Pandarost, the planet where the Master had prepared his trap for the Doctor and Izzy. Both were caught in the Church of the Glorious Dead's assault and barely survived, living a hard existence for nearly a month, fending off attacks by cultists and their monstrous Ash Wraith forms.
Kroton finally did battle with Sato Katsura, lasting against him much longer than any other foe due to his enhanced physiology. Sato was sent reeling from the fight when Izzy forced upon him a mnemonic crystal and fed him back all the pain she had suffered at the hands of the Church. She gave the same crystal to Kroton, who used it, though he had rejected it before.
It awoke memories from before his Cyber-conversion. He remembered all the happiness and sorrow he had ever felt as a human being. He accepted them and chose joy over misery. He was chosen as the new centre of the Omniversal Spectrum, banishing the Master and restoring the history he had warped, granting Sato his ultimate wish for honourable death as he left for the heavens. (COMIC: The Glorious Dead)
Iris Wildthyme once claimed that Kroton was a friend of hers. (PROSE: The Scarlet Empress) An image of Kroton appeared on one of the Knowsall screens displaying the Tenth Doctor's memories. (COMIC: Thinktwice)
Early on in his life he showed a child-like innocence to everything around him. He was often bothered and horrified by death or tragedy that surrounded him. (COMIC: Throwback: The Soul of a Cyberman, Ship of Fools) In his later life, however, he grew more cocky and confident.
Kroton appreciated beauty. Upon leaving Telos for his mission on Mondaran, he considered out loud if he would ever see the skies of his home planet again. (COMIC: Throwback: The Soul of a Cyberman) These qualities made the Doctor realise Kroton was no ordinary Cyberman. He also withstood the impression of the interior of the TARDIS; when the Doctor asked him if it struck him as "illogical", Kroton happily cast away the word, stating, "To hell with logic!". (COMIC: The Company of Thieves)
He had a balanced sense of right and wrong, becoming utterly infuriated at the possibility of a Sontaran plan for genocide of innocent natives and the Church of the Glorious Dead's atrocities against the alien ambassadors on the planet Paradost. (COMIC: Unnatural Born Killers, The Glorious Dead)
Due to helping out a group of Technosmiths on Baroq VII during his journeys, they thanked him by making some "special adaptions" to his armour — which allowed Kroton to withstand blaster fire from various guns, including Sontaran lasers, as well as withstanding a powerful electric charge after the Eighth Doctor mistook him for a normal Cyberman and shoved a high-voltage cable into his chest unit — which merely disabled him temporarily. He even stated to the Doctor when revealing the adjustments that "when half the galaxy keeps trying to shoot you on sight, you learn how to protect yourself!" (COMIC: Unnatural Born Killers, The Company of Thieves)
He had a metal staff that he could use during combat as well as magnetise it to his back. Kroton was seen using it to great effect during his battle with the Sontarans and the Space Pirates before finally using it during his battle with Sato. (COMIC: Unnatural Born Killers, The Company of Thieves, The Glorious Dead)
Behind the scenes
- Created in 1979 during the early issues of Doctor Who Magazine, Kroton's story was initially meant to be a one-off, but his story saw brief continuations over the years. This eventually ended with him becoming a brief companion of the Eighth Doctor, more than twenty years after his creation. The character's creator, Steve Moore, had said that he was not aware of the character's use outside of Ship of Fools, and that he thought of that later stories to be too far a removal of the initial concept.
I probably wouldn't have approved. From what little I've seen of this stuff, it looks to be a complete transformation of the original character... he's now less a "Cyberman with a soul", more just a human being in a funny suit. It's far too far a move away from the original. If I'd done anything else with him, it would have been to continue with the character pretty much as he was... only a little different from a normal Cyberman, and still very much the "innocent abroad" that he was in Ship of Fools. The later characterisation just seems far too knowing to me. But I don't know if I ever would have done anything else with Kroton... I never really thought that far ahead at the time. 
- Kroton isn't the only Cyberman companion; the next would be Handles, who appeared in the television story The Time of the Doctor and in short stories in Tales of Trenzalore— though Handles is not (or no longer) a cyber-conversion victim, but the head of a Cyberman with all Cyberman protocols and organic parts cleansed out. The Twelfth Doctor's companion Bill Potts would also later undergo cyber-conversion in the television story World Enough and Time.