Wilfred Mott
Although a general rule for this article's definition of recurring character is that the character cannot be the family of a companion, Wilf gets an exception because his role in the show is not purely defined by Donna. While Sylvia Noble is technically in more episodes than him (because of the 60th anniversary specials), she is only ever depicted as Donna's mother.
Wilfred Mott was first introduced in the 2007 Christmas special "Voyage of the Damned," where he ran into the Doctor for the first time as one of the only people to stay in London for Christmas. He loved the idea of space and aliens, encouraging Donna to go with the Doctor and see the stars. In addition to his Series 4 appearances, Wilfred was the primary companion during both halves of "The End of Time" and made a brief appearance in the 60th anniversary episode "Wild Blue Yonder" before actor Bernard Cribbins's death.
He appeared in 10 episodes of Doctor Who, along with a few non-televised appearances in the wider Doctor Who universe. Despite his connections to Donna, Wilf had his own relationship with the Doctor. He often appeared when the Doctor was facing a moral quandry, representing the best of humanity. In a time of the show when human companions frequently became the most important people in the universe, Wilf was just an ordinary, good man.