I already answered that on your profile wall but lets get more into it.
I play a lot of (casual) Kotal coz that's the only character in this game i semi like (less than semi thou). Rain is one of the easiest MUs for Kotal. You can't zone Kotal with Rain coz of parry and sun, there is no way you gonna outdamage the sun. You can try to bait him into doing sun when youre near the end of the screen but not quite there, you should be able to dash in and punish but most decent Kotal players wont do it unless they know you cant get there in time or you're low on health that even trading will do it for them.
There is no playing midscreen game vs Kotal, F2 and B2 just cover all the options and amp disc on block leaves Kotal (at -1 on block so it's nothing) in a perfect spot to do b2,2 and punish you for trying to punish him. So yes you should try to play up close, the problem with this is obviously cmg. He can do it of canceling d1; 1 or 1,2 the last two are quite important coz it's a 3 way mix, he can do 1 into cmg, 1,2 into cmg, 1,2,2 (there is a FBlock gap here but again it's a guess - if he even goes for 2 or not) and that's +1 on block with gaint pushback (ideal for him) or if he thinks you gonna try to jump out of cmg he can just do 1,2, cut which will lead into full combo (thou if he get it wrong and you block it - it's full combo punishable). If you get hit - you can breakaway just before the cmg comes as it has billion frames of recovery so you can punish him for it (thou most Kotals just stop before the cmg if you have 2 bars of defensive meter coz the extra dmg from finishing with cmg isnt worth it).
With Rain Kotal like pretty much any character can take a lot of chances, coz Rain dmg is laughable while if you get hit by Kotal which always end in cmg it starts from 330 and goes up (quickly) from there. Kotal has some not great MUs but Rain isn't one of them.
@up Also Aztec isn't the best Kotal player, Turk (alias Turkey) is by a mile. I advice you to watch Aquamans Kotal only tournament on YT or Twitch where they both played.