I have to characterize a female character and in my text appears the following sentence ''She wasn't a cat.'' What do you understand by ''She wasn't a cat.'' How is a cat?
As you know from another thread, there are several idioms involving cats, and the book could be referring to any of them. A cat 'always lands on its feet'. walks quietly ('with cat-like tread'), 'has nine lives'.... Cats also have well-known characteristics: loyalty to places rather than people, agility, selfishness, haughtiness, toying with their prey, killing when they don't need to, fastidiousness (some cat-lovers may disagree, but others value these traits). The book could be referring to any of these.
Incidentally, you can ask a specific question using 'how': 'How is your/the/that cat?' [=is it well?], but if you want to know a cat's general characteristics you should ask something like: 'What's a cat like?' or 'What's special about a cat?'
That's another figure of speech I hadn't thought of: 'catty' (spiteful and unpleasant). There's also the expression 'cat-fight'. And the 'spiteful and unpleasant' idea has even entered the language of gestures. When someone has said or done something catty, you can mark it visually by snarling and making a clawing gesture, possibly - but not necessarily - accompanied by the word Miaow.