Ren’s not much for combos because of the way her normals work. Here’s some of the ones I use.
2+A > 2+B > 5+B > j. A > j. C > dj. A. > dj. C > airthrow
You can add in another 2+A at the beginning if needed. Make sure 5+B hits 3 times instead of 2, or else they’ll be able to flip out faster than usual. You can also use j. B instead of j. C, but j. B comes out a few frames slower and doesn’t work regularly for me. If you’re in the corner, tiger knee her air EX Ice after the airthrow. It’s unavoidable if they groundtech and they’ll eat full damage, and if they don’t groundtech the combo will continue (but be invalid and deal less damage). You can also add in a j. C > airdash j. C at the beginning.
2+A > 2+B > 5+B > EX Ice
Basic EX Ice combo. Some variations include removing the 2+A, or doing two 2+B, which scales the damage quite a bit but will set them up for the BnB listed above. Never use the EX Ice outside of a setup or combo; it will lose against just about anything in the game.
Air EX Ice is good if you land it off a j. C near the corner; it’ll juggle them up, leading to traps with ground EX Ice if they airtech. If they don’t tech, you can EX Ice and then land her BnB afterwards (2+B > 5+B > j. A > etc etc etc)
I’m not sure if you can combo off her airthrow anymore. I’ve been able to land a 5+A after a high airthrow provided Ren lands first, but it’s rather difficult to do.
Prime punisher for a whiffed move would be 5+C (5 hits) > 3+C > EX Ice or A Ice if you lack meter. Ren’s 5+C must be landed very close or else not all the hits will land, and you can only buffer it if the last 3 hits connect.
Ren’s got some good mixups due to her low jump and short airdash. Jumping B and C are both easy crossups. Jump C, airdash C, dj C is one pattern, you can also IAD and throw, IAD C for instant overhead, IAD B (won’t come out in time) to fake a crossup/instant overhead. Counterhitting in the air off a jumping A will give you a free combo, and jumping A has decent range and good speed. 2+B also has deceptive range and cannot be teched at all, but you can’t Ren or Satsuki due to their character gravity.
That’s all I can think of for now. 
