Radu Gogorsky is the biological father of Lois Wilkerson, ex-boyfriend of Ida Welker, and possibly the father of Susan Welker. He has never been seen in the show, but was mentioned in the episode Victor's Other Family.
Radu Gogorsky was a man who dated Ida Kenzel for a short period of time. The only known fact about their relationship is that Ida liked him because he could climb an entire flight of stairs with his hands. During one of their times, having sex, he must am impregnated her, with a child that would later become Lois Wilkerson. Shortly afterwards, the two must have split up and Ida moved on to a man named Victor Welker, who would become the man who raised the child.
Throughout all of her childhood and most of her adulthood, Lois thought that Victor was her real father, until the events of the episode Victor's Other Family, where Lois received a blood test that was done on her family, that revealed the truth. This even came as a surprise to Ida, who could have sworn that it was Victor who impregnated her. As for Lois' sister, Susan Welker, even Ida doesn't know the full story behind that, leaving it ambiguous as to whether Susan's real father is Victor or Radu.