Otto Kreiner

Otto Kreiner was the brother of Jürgen Kreiner, the husband of Muriel Tarr, and the father of Fitz Kreiner. He was born in Germany, but fled to England to escape the Nazis. When World War II broke out, he faced persecution for his nationality. (PROSE: The Taint) Due to his treatment by the British, Kreiner sometimes wished that he'd never left Germany. Otto had a friend from home named Franz-Joachim, and he told his Fitz that if he had stayed in his homeland, he would have named him Franz-Joachim Kreiner. (PROSE: Frontier Worlds)

Behind the scenes
In Dale Smith's Recursion, a short story published in Myth Makers Presents: Essentials in 2003, it is revealed that Otto was killed during an alien excursion that the Eighth Doctor, Fitz, and Anji Kapoor failed to stop. Fitz and the Doctor attempt to change the past several times, creating many paradoxes, before a future version of Fitz who mirrors Father Kreiner prevents the earliest versions from ever trying to save Otto.