The first thing we learn about Jan is that he is a "clever orphan boy." He is very secretive, refusing to tell Joseph his surname and insisting that he is just "Jan." We therefore see that he is street-smart and well aware of the dangers around him. We see more...
of his secretive nature later. After he has "made his home" with Ruth and Bronia, Jan refuses to reveal the contents of his wooden box and carries it wherever he goes for safekeeping.
Jan is defensive and hates all soldiers with a passion. We learn of Jan's hatred for soldiers when Ruth catches him attacking a soldier with a knife. He ignores her instructions to stop and cannot understand how any soldier—whether Nazi or not—could be considered a friend.
It is evident that Jan is brave and that he is a protector of sorts. When discussions begin about the trip to Switzerland to find the Balicki children's parents, Ruth expresses gratitude that Jan wants to join them, as she values his ability to keep them safe. Jan has shown himself to be courageous and unafraid in the face of danger.
Another characteristic of Jan's—and one that you could describe as surprising—is that he cares for animals. When Ivan suggests that they make a meal out of Jimpy the cock, Jan declares Jimpy to be his friend. Throughout the long journey, Jan takes care for Jimpy as best he can, eventually "carrying Jimpy's limp form."
In what I would call another surprising twist, Jan reveals himself to be an optimist. During the train ride from Posen, Bronia complains that the train "jolts too much" and offers "no room to stretch." Jan points out that its better than the other trucks and that it has a stove that can be lit for warmth.
In a nutshell, Jan is a street child who has been left to look after himself on the streets of Warsaw. This has made him quite wild and adept at surviving in tough circumstances. He is a good fighter and a talented pickpocket, stealing food from Joseph's pocket without Joseph having a clue. He hates the Nazis and even successfully steals from the shop in the Nazi barracks.