Yankel is Jonathan's spiritual ancestor and namesake. No, he isn't really Jonathan Yankel Foer, but Yankel was named Safran before whatever happened to disgrace him, so that name, too, becomes part of this family's history. He's the one chosen to raise Brod after she magically bobs to the surface.
The shtetl is later named Trachimbrod in honor of this accident, and Brod is raised by the disgraced usurer Yankel D. A usurer is pretty much a loan shark. So, already we're off to a bad start.
It doesn't get much better, either. Poor Yankel has had a hard life. His actual children died of illness, and his wife just up and left him one day. Unable to tell Brod that her "mother" left, he creates a fake backstory about her, even writing fake letters from her. They're so convincing, he almost believes them to be true. When he starts to lose his memory, he writes them all over the walls in lipstick. He lives in this fantasy world, half-real/half-not, up until his death one fateful Trachimday, when he dies clutching a note that says Everything for Brod (13.81).
That note tips us off that Brod is his whole life, and he always does his best to be a good father … we think. He buys her books even though they can't afford them and educates her, speaking "as if she could understand him […] never in nonsense words" (7.1).
Aside from probably giving her an awesome vocabulary, that tactic makes her wise beyond her years—which turns out to be just the skill she needs to survive in Trachimbrod.