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He was particularly hard hit by Fievel's disappearance in the first film, coming to the conclusion that Fievel had drowned. He would hear none of it whenever Tanya begged him not to give up hope because she felt Fievel was alive somewhere. However, he is overjoyed when he is the first to be reunited with Fievel at the end of the movie at the Orphan alley. He was always a strong believer in the American dream, and he shows many of the same adventurous, optimistic and aspirational ways of thinking that Fievel and Tanya develop. So yes, this is where they get it from. And occasionally, he is taught to be more optimistic by his children.
His first name, Bernard, is only revealed in Fievel's American Tails.
Papa is obese and tall, with a gray beard, tan shirt and brown pants, with small glasses on his snout. After giving Fievel his hat he never wears another one. His appearance changes very little in Fievel Goes West, besides being animated in Amblimation's style rather than Don Bluth's.
In the direct-to-video sequels he looks more scruffy and worn down, perhaps because of the terrible working conditions at the cheese factory aging him. He is missing teeth and has an untrimmed mustache.
You can find Papa Mousekewitz gallery here.