You know me as RisingShieldBro online.
Speak of the devil..I don't see this MU being very good for Rain. I haven't personally played this MU but I do play both characters to some degree. I'd say try to stick as close to Kotal as possible and exploit the fact that his mids and pokes are (relatively) slow. He still has tic throws and his D3/D4 are great but imo once you get into the mid range or full screen Kotal has the advantage. His normals are so huge and some with disjointed hit boxes and receded hurt boxes that make them hard to whiff punish, and they all lead to a lot more damage than Rain can get. From full screen he can put up sunlight and besides argus plunge amp or the KB idk what Rain has that can even make it so he doesn't heal more damage than he takes. He can also parry your katars while buffing his own damage and even though you can duck disc or evaporate one mistake can you eat a KB that does the same damage as a regular Rain combo.
EDIT: A bit more info on sunlight - always try to punish it if they do it in the neutral. It heals 105 over 10s so your aim is to do more than 105 to negate the effects.
Here's DJT VS Aztec (probably the best Rain player VS probably the best Kotal player):Hope this helps