The Question and Answer section for MAUS is a great resource to ask questions, find answers, and discuss the novel.
The characters are mice but more symbolic than literal. Artie interviews his father, Vladek, about his experiences in world war 2 in order to make a graphic novel.
Haskel is Vladek's cousin, and chief of the Jewish Police in the Srodula ghetto. He is the brother of Miloch and Pesach. He is what Vladek calls a kombinacya, or "schemer." While he is a rather unsavory character, he is a good person to know in...
Are you referring to the book Maus? I don't recall Hitler in WW1. Still, you can check out this link: