Symbolism, Imagery, Allegory
A big deal is made about Peter's kitty. Peter's sort of weirdly obsessed with the thing, while Anne is sad she wasn't able to bring her own cat. Mr. Frank says, "I'm glad you brought your cat" (1.2) and gives it a box to sleep in. On the other hand, Mr. Van Daan really, really hates it, and threatens to get rid of it in practically every scene. Eventually, Mouschi mysteriously disappears.
What happens to Mouschi is eerily similar to the situation of the Jews in Holland as well as the Annex residents. Many Jews would disappear each day during the Holocaust, never to be heard from again. Mr. Van Daan's attitude that the cat was eating up their precious resources is similar to that of the Nazi viewpoint about the Jews.