You might like to consider the following adjectives as suitable traits for Jacob Jankowski. Do remember, however, that characters can change during the course of the book, and the Jacob Jankowski that we see at the end of the story is very different from the young, innocent man who first sets out on the train. I would say that Jacob Jankowski is: innocent, compassionate, a man of integrity, brave, hardworking and loving.
Note how innocent Jacob is when he first starts working for the circus. He is entranced by the sexually charged performance of the prostitute in the circus and is hopelessly inept with women. However, he is also immensely compassionate, with both humans and animals. Note the way that he has to kill Marlena's favourite horse and how it sickens him. He is also a morally upright individual who does his best to stand up for what is right. This is shown in the way that he is revolted by how the elephant is mistreated for no reason. His bravery is shown in the way that he is willing to fight for Marlena and oppose her husband. His hardworking, industrious nature is shown from the first day in the circus, and lastly, his loving nature is illustrated through both his close relationship with Marlena but also the animals that he looks after.