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. He served at Ypres in December 1914 and, following an encounter with the Twelfth Doctor and First Doctor, participated in the Christmas Armistice. Before taking their leave, both incarnations of the Doctor assured the captain that they would check up on his family, (TV: Twice Upon a Time) leading to the Second Doctor's fateful encounter with his relative Alistair Gordon Lethbridge-Stewart, who would become a long-time friend of the Doctor. (TV: The Web of Fear et al.)

Behind the scenes
While not stated in the episode, Mark Gatiss, who portrayed the character, confirmed that Archibald is supposed to be a grandfather of Brigadier Alistair Gordon Lethbridge-Stewart. Previous sources named the Brigadier's paternal grandfather as Alistair Lethbridge-Stewart, father of Gordon Conall Lethbridge-Stewart.