Who is Grandpa Portman (Abe) from Miss Peregrine's Home for Peculiar Children and what is their importance?

Abraham Portman is Jacob's grandfather, a man who, as is eventually revealed, was a Jew in Poland at the beginning of World War II (see "Setting"). He was also, again as the narrative eventually reveals, a "peculiar" (see below), a young person with an unusual ability - in Abe's case, the ability, which he shares with Jacob, to see the hollowgast (again see below), a monster that feeds on peculiars. Abe's stories of his unusual childhood at first entertain the young Jacob who, as he grows older and somewhat more worldly, comes to regard them as fairy tales. Interestingly, Abe never confronts Jacob about his disbelief, choosing to let him simply mature at his own rate and form his own opinions. The narrative never makes it explicitly clear why Abe makes this choice. There is, however, the distinct possibility that, as the result of living in Miss Peregrine's home, in which the lives of the children living there are strictly, almost rigidly, controlled, Abe developed a lifelong aversion to being too much of a dictator to the young people in his care, letting them be who and what they wished. This might, in turn, add another layer of explanation about why Abe was such an absent father to his son (Jacob's father) - yes, as the narrative makes clear, Abe was hunting down and killing wights and hollowgast. But it's also very possible that having been controlled as much as he was by Miss Peregrine, and having escaped the control of the Nazis, Abe wanted to give his family as much freedom as possible. In any case, Abe Portman was a warrior, an aspect of his identity long kept secret but which, when eventually discovered, at first intimidates but then inspires his grandson.