Ko Sharmus

Ko Sharmus was one of the last remaining humans left to guard a dimensional gateway at the tail end of the Cyber-Wars. (TV: Ascension of the Cybermen) A general in the Alliance that sent the Cyberium back in space and time, Ko Sharmus later sacrificed himself to stop and his army of CyberMasters with the death particle. (TV: The Timeless Children)

Cyber-Wars
Ko Sharmus fought in the Cyber-Wars against the Cybermen. His first commanding officer often told Ko Sharmus to "be brave, be swift. Most of all, be lucky." Ko Sharmus eventually rose to the rank of general in the resistance unit known as the Alliance. As part of the Alliance, he was one of the people responsible for stealing the Cyberium from the Cybermen and sending it back through time and space to hide it. (TV: The Haunting of Villa Diodati, The Timeless Children)

Having fought in the war for "a long time," Ko Sharmus was eventually captured and sent to a Cybercamp for processing and upgrade into a Cyberman. Along with a handful of others, he managed to escape and set up a settlement on a planet containing the anomaly known as the Boundary, something that they had heard tales of but didn't really believe in. (TV: Ascension of the Cybermen)

Ferryman for survivors
As the war had not yet reached the planet where Ko Sharmus had settled, they were safe and word eventually spread causing others to join them. Ultimately, the others fled through the Boundary, but Ko Sharmus remained behind as he felt like it was his duty to wait in case there were other survivors that would come. Over the years that followed, Ko Sharmus and the Boundary became a legend with Ko Sharmus eventually being forgotten as a person rather than a place. With humanity and the Cybermen both driven to near-extinction by the war, people eventually stopped coming for so long that Ko Sharmus came to believe that there were no humans left alive in the galaxy. (TV: Ascension of the Cybermen)

Return of the Cybermen
After an attack on the refugee planet by Cybermen led by Ashad, the survivors decided to seek out Ko Sharmus and the Boundary. Ko Sharmus was shocked when the Thirteenth Doctor made contact with him, having long thought he was the only human left. Ko Sharmus explained some of his history, surprising the Doctor with his selfless actions. Ko Sharmus showed the group the Boundary which he called "the last way out of this galaxy" and preferable than the other options to those who stepped through it.

The group received a call from Yaz warning that she and the others were approaching in a Cybercarrier with Ashad and a lot of awakening Cybermen. As the Doctor tried to get Yaz to flee, the Boundary suddenly shifted to show Gallifrey which shocked Ko Sharmus who had never seen the anomaly look like that before. As they tried to understand what happened, suddenly arrived through the Boundary. (TV: Ascension of the Cybermen) By threatening the lives of Ko Sharmus, Ryan and Ethan, the Master was able to force the Doctor to join him in traveling to Gallifrey while the humans remained behind.

As the three attempted to decide on a course of action, the Cybercarrier arrived with its army of hundreds of thousands of Cybermen. Anticipating an attack, Ko Sharmus led Ryan and Ethan to his weapons store and insisted that they had to fight to survive when Ryan hesitated at the idea of using weapons. As Ko Sharmus expected, Ashad dispatched death squads to hunt down the humans on the orders of the Master. Using automated turrets and a bomb thrown by Ryan, the group managed to take out most of the Cybermen before being forced to flee inside by the advancing forces. Splitting up, the three managed to take out a few Cybermen before Ethan was captured and his life threatened to force Ko Sharmus and Ryan to surrender. The three were saved at the last moment by Yaz, Graham, Ravio and Yedlarmi who had escaped from the Cybercarrier by disguising themselves as Cybermen.

With the Doctor, Ashad and the remaining Cybermen on Gallifrey, the group decided to cross the Boundary to help her. Finding the Doctor, they explained that they had brought enough explosives to blow up the Cybercarrier and Cyber-Army aboard it, but the Doctor warned them that it wouldn't stop the Master and his army of CyberMasters. The Doctor remembered Ashad mentioned the death particle which would wipe out all organic life and thus could be used to end the Master and his army. On the Doctor's orders, the group planted the explosives throughout the Cybercarrier while she recovered Ashad's shrunken remains. However, the main trigger device unexpectedly activated forcing them to flee the ship with moments to spare as the Cybercarrier and the army of hundreds of thousands of Cybermen aboard it were destroyed, foiling Ashad's plan to rekindle the Cyber-Empire. (TV: The Timeless Children)

Sacrifice
The Doctor led the humans to a TARDIS still present on Gallifrey that she set to take them to Earth in the 21st century. However, Ko Sharmus only possessed a single explosive which could only be detonated by hand, meaning that someone would have to sacrifice themselves to stop and the CyberMasters with the death particle. However, Ko Sharmus decided to follow the Doctor at the last moment rather than leaving with the others. As the Doctor hesitated to detonate the death particle, Ko Sharmus intervened, revealing himself to be one of the people who sent the Cyberium back through time and space which caused Ko Sharmus to blame himself for everything that had happened. Ko Sharmus insisted on being the one to sacrifice himself to make up for his actions and the Doctor fled, escaping in another TARDIS. The Master furiously ordered Ko Sharmus killed, but Ko Sharmus taunted that he had killed the Master first. On the Master's order, the CyberMasters blasted Ko Sharmus multiple times, killing him. In his last moments Ko Sharmus was able to detonate the explosive, setting off the death particle and wiping out all organic life on Gallifrey, presumably killing the Master and his CyberMasters in the process. (TV: The Timeless Children)

After being reunited with Team TARDIS, the Doctor told Ryan that all life on Gallifrey had been destroyed thanks to Ko Sharmus and the death particle. (TV: Revolution of the Daleks)

Personality
In general, Ko Sharmus appeared to be a kind and gentle man with an extremely friendly personality.

Ko Sharmus was a warrior who fought for humanity during the Cyber-Wars, something he retained even in old age. He showed himself to be extremely selfless, effectively sacrificing his own life by remaining behind as a ferryman for those who sought to use the Boundary to escape, even when it had been so long that he believed humanity had gone extinct aside from those who had already escaped. However, Ko Sharmus didn't appear to find this as noble as it sounded from his response to the Thirteenth Doctor when she pointed out how selfless he was. Though he hoped he'd fought his last battle a long time before by the time the Doctor found him, Ko Sharmus still retained his warrior spirt and claimed to still know a few tricks and expressed confidence in his knowledge of the Cybermen.

As part of the Alliance that sent the Cyberium back through time and space, Ko Sharmus felt a great deal of guilt and remorse as he believed himself to be responsible for Ashad's actions and by extension, the army of CyberMasters created by. Ko Sharmus felt that by saving the Doctor he could in some way make up for his mistake. In his final moments, Ko Sharmus showed nothing but defiance in the face of the Master, taunting him with his last words. (TV: Ascension of the Cybermen, The Timeless Children)

Appearance
to be added