Archibald Hamish Lethbridge-Stewart

Captain Archibald Hamish Lethbridge-Stewart was a member of the Lethbridge-Stewart family who served in the British Army in World War I. (TV: Twice Upon a Time) He had a brother named Alistair, who was the grandfather of Brigadier Alistair Gordon Lethbridge-Stewart. (PROSE: Night of the Intelligence)

History.
He served at Ypres in December 1914 where on Christmas, Archibald was stuck in a crater with a wounded German soldier. As the two men held each other at gunpoint, Archibald stated that he wouldn't kill the German soldier except in self-defence but recognised that they couldn't understand each other.

In the moment before they killed each other, Archibald was abducted by the Testimony in order to record his memories. Due to an error in the timeline caused by the First Doctor and the Twelfth Doctor resisting regeneration at once, Archibald was returned to Antarctica in December 1986 instead of Ypres. Archibald became the subject of a conflict between the two Doctors and the Testimony who wished to return Archibald to his own time. At one point, Archibald stated that while he was unafraid of death before, now that he knew he would die, he was scared again.

After the truth about the Testimony was revealed, the Twelfth Doctor agreed that Archibald must be returned to his own time period. However, the Twelfth Doctor requested that he and his predecessor do it as it was their fault that Archibald had been displaced in time. On the return trip aboard the Twelfth Doctor's TARDIS, Archibald expressed regret that he would never see his wife and sons again.

After returning to Ypres, Archibald was informed that his memory of the entire experience would be erased while a perception filter would keep him from seeing the two Doctors. The First Doctor expressed regret that "the universe generally fails to be a fairy tale" and Archibald asked the Doctor to look after his family which the First Doctor promised to do. Archibald revealed his name, shocking the Twelfth Doctor as he knew the man's family. Both Doctors promised to keep an eye on Archibald's family and he returned to his spot in the crater, causing time to resume and Archibald's memories to instantly be erased.

As Archibald and his German opponent prepared to shoot each other, both men heard Christmas caroling as the Christmas truce began. Laughing, Archibald and the German soldier ended their standoff and Archibald called for medical attention for the man. Watching unnoticed, the Twelfth Doctor explained that he adjusted the time period Archibald was returned to so that it was a couple of hours later when the truce began, saving both men's lives. The Twelfth Doctor's meddling in time to save both men caused the First Doctor to finally understand what being "the Doctor of War" meant.

Shortly before regenerating, the Twelfth Doctor was noticed by Archibald. The Doctor gave a confused Archibald a salute which Archibald returned. (TV: Twice Upon a Time)

Alternate Timeline
In an alternate timeline, Archibald and his German opponent killed each other on the battlefield. (TV: Twice Upon a Time)

Behind the scenes
While not mentioned in the episode, Mark Gatiss, who portrayed the character, stated that Archibald is supposed to be a grandfather of Brigadier Alistair Gordon Lethbridge-Stewart. Which was not denied by character's creator Steven Moffat.

However, previous sources named the Brigadier's paternal grandfather as Alistair Lethbridge-Stewart, father of Gordon Conall Lethbridge-Stewart and the novel Night of the Intelligence names Archibald as the brother of the elder Alistair. Indeed, Andy Frankham-Allen made it clear that neither the Haisman nor Henry Lincoln Estate, copyright owners of the Lethbridge-Stewart IP, consider the character as the grandfather, but that he was rather the grandfather's brother, as previously named in the novel.