Katt is a gladiator at the coloseum, and you'll have to whup her when you first meet. She'll have joined you by the time you're done in Coursair. She'll leave you after you triumph at the Thieve's Tomb, but she'll rejoin you once her reason for leaving you gets eliminated.
Fighting/Defense -- Fighting is all that Katt can really do, and she's pretty good at it. At first, she hits as hard as Ryu. He'll pull ahead eventually, though. Katt has relatively low HP and defense, however. Katt can counter-attack. Katt's best combate attribute is that she'll usually make the first move in any given combat round -- her speed is very, very high. Her weapon is the staff. She's somewhat restricted in armor.
Magic -- She's the worst of all the characters in this area. She'll learn Hail, Fireball, and BoltX, but she won't have enough AP to cast even Hail until she's around level 40 or shamanized. The only bonus spell you should consider giving her is Chop-Chop.
Special -- Dare. Katt forgoes all action in favor of attracting attack. She's much more likely to counter-attack when doing this, but I sincerely reccomend not using it -- she might not survive the round. Only use it if either 1) you think she'll dodge most of the attacks, or 2) you think she won't get killed.
Shamans -- Devil alone or Devil + Water, Fire, or Wind, turns her into the Ninja Cat. She gets a massive attack boost, and comes closer to having a decent defense rating -- and, of course, there's the vigor boost. The AP boost is just enogh to grant access to Hail and perhaps Fireball, but she can only make one casting before exhausting her AP. Her special changes to Keep. She does nothing for one round, and the next round she'll automatically attack for around double her normal critical hit damage. Only Ryu's Dragon attacks can top this.
Field -- Katt can hunt. If you see a spot of grass that's blowing in the wind while wandering the world, you can "talk" to it to enter a hunting ground. Put Katt in the in the lead and press Y to swing her staff. Once you connect with something, just keep swinging. For all practical purposes, Katt can't catch Deer or Birds, so don't bother trying to get them. Boars and Deer drop Roast when hit twice, and flee when hit once. Bears drop Spare Ribs when hit 8 times, and will charge when damaged. Birds drop Egg when hit. They'll appear after something is killed, so be be watching. Beaks die after 6 hits, and drop nothing -- though they can "steal" (eat, really) stuff that's you haven't picked up yet. They charge whenever they have a straight line to you. If you kill a Beak, expect another one to appear eventually. Trying to occupy the same space as a Beak or Bear results in a fight. You can't hunt again until a new spot appears. There are a few spots in the game where you'll need Katt's staff to proceed; she can shatter rocks and a few obstructions with it. She can't hunt in shamanized form.
At level 40 -- Her shamanized form is devastating, but just plain Katt ain't. She's slightly above the average in attack power, but her defense still sucks. She's still the fastest character you've got. Capatalize on that fact, and you'll be able to get the most out of her. Just remember to watch her HP, because it's still pretty low.