Theodore Block is a one-time neighbor of Jon Arbuckle, he was known for blindly hating cats.
Mr. Block initially has a severe case of ailurophobia - he hates cats with such passion, that he checks to see if his guests are feline. This can be seen when a TV talk show host tries to invite Theodore to his talk show. The background of his prejudice is revealed after Garfield sneaks into Mr. Block's attic. There, Garfield discovers that Mr. Block, as a child, lost faith when his only friend- a black and white kitten named Foo Foo- disappeared without a trace.
His stance changes dramatically after an adult lookalike of Foo Foo is presented to Mr. Block. He instantly abandons his long held hatred towards cats.