Foo Foo was a kitten, who belonged to Theodore Block when he was a child. In The Man Who Hated Cats episode, Garfield managed to find an older cat who looked similar to the lost original, who took the name of the lost original and cured Mr. Block from his ailurophobia.
In The Man Who Hated Cats , it is mentioned that Theodore Block didn't have friends in his childhood. This changed, when his uncle Steve gave him a black and white kitten as a Christmas present. Young Theo and the kitten named Foo Foo spent plenty of time with each other, until Mr Block's cat suddenly disappeared without a trace.
Having discovered, what happened to the original Foo Foo, Garfield managed to find a stray lookalike in the downtown alley, who eventually agreed to live with a human.
Not much can be said about the original Foo Foo, since he's only mentioned by Theodore Block. According to his memories, the kitten was cheerful, friendly and devoted to his owner until it disappeared.
Foo Foo's adult lookalike shares the kitten's friendly attitude and cheerfulness (though he needs stimuli to perform those, such as promise of living in warm and dry conditions) and gets accustomed to his new owner. As for stray cat, the lookalike tends to behave in rather clumsy way, which can be seen in dancing scenes with Garfield and while they approach Mr. Block in TV studio.