if natsu doesn't knock me down, and keeps doing fast horizontal natural combos, how do I put some distance between me and her?
specifically things like her aa, kkk, kk2k, they are fast, and after block those strings, even if I am given advantage, I can't seem to do anything beside grab, 2a, 6b, and 2k because she can hit me before my aa even comes out. I don't seem to have fast moves to do after guarding close range attacks because even my natural combos like aa and bb takes time to come out compare to something like pyrrha's aa.
Well, I will tell you this: you don't want to attempt to try to beat her out close because Natsu is a rushdown character meant to shut down people, specifically up close. Her up close game is probably one of the best in the game. If she gets in, you'll be doing a lot of blocking. The pokes you mentioned are your best bet, then blocking. 6B by far is going to help you out in this fight. If you seen my earlier post with Pyrrha, successfully landing a 6B can allow a safe option to safely deal with quick counterpokes that the opponent THINKS would be able to punish a blocked 6B attack. Like I said with Pyrrha before, this opens up an opportunity to just guard the counterattack. For fast attackers like her, the key up close to them is to get them to see you can just guard. Smart players will back off you, or at least attack a little less because they recognize that you can build meter for free just guarding. Scrub players will continue to mash, in which case you can continue just guarding, building meter, and GI their attacks, and followup with a 1A as a surprise sweep move to open up lots of space between you and the opponent.
Once they see you can just guard the counterattack after attempting to punish your 6B, they(again, if they are smart) will naturally back off of you themselves to keep you from building meter on them for free, or at least attack a little less. This is your opportunity to control the mid-long range field by using 22_88AA to stop steppers. If you need a faster kill step option, go for 3A~B. This is also not a bad up close attack, but you must place it carefully because Natsu may be able to beat it with her quick A attacks.
I'm not sure if this is well known knowledge already, but I have implemented a quick backdash technique with Xiba that allows him to just guard at times while quickly retreating. All you have to do is learn the just guard technique. Once you do, you can do 4+G as if you were just pressing G alone for a just guard. Keep doing this in quick succession, and you'll find yourself doing sort of a backdash cancel technique mixed in with a just guard command! This is something I do a lot and it does wonders for me with good up close characters like Natsu, and puts space between you and her nicely while guard guarding in retreat. This can REALLY confuse and mess up Natsu, and other players in general, which is exactly what you want! In short, implement just guard to gain the psychological edge to back them off you! :-)
Update: Since the patch update, the aforementioned backdash technique no longer works. If you try it, you won't be able to cancel as many times. I still use it just as a means to keep my opponent off balance, but be wary that you cannot block during a backdash, so use it carefully and wisely.