Left 456 Right
Punch = P
Kick = K
Free = F
You want to avoid an upclose game at all costs. She is too fast and will put you in a deep critical stun very quickly which you want to avoid. One stun can be the entire round and I stress when I say one stun. What you want to do is keep kasumi at a middle distance where all she can really do is 6p or 6k to try and get in. Everything else is too slow or risky. The biggest issue right here will be 6k which stuns and on counter hit gives a ground bounce for a juggle.
In this distance your best options are safe pokes that keep her at bay like crouch jab or a 3k which usually has a lot of range and is quick for most chars or 3p if you are say Ayane. Can also use long ranged highs as well because Kasumi's jab while being fast has no range. 6p will counter hit some moves so be careful however because of the distance she has to follow up with a string she can't really extend the stun since she will be so far away.
What you will also want to do is guess counter 6k. I recommend waiting for any subtle movements and then hold. This of course will deal a lot of damage depending on the character you have. Chars with good jumping mid kick holds are Gen Fu, Hayate and Kokoro for example. As long as you keep Kasumi out you will win.
Remember if she gets in then starts all her ridiculous deep critical stun string pressure into okizeme nonsense you will most likely lose because it is a problem trying to keep Kasumi off of you when she gets in. With a launching 6f+p and a crouching grab too that leads to juggles and her ridiculous on the ground game you might want to lay off trying to hold and take the critical combo because you basically give her bonus damage. Also don't forget about her teleport parry. Try not to get too predictable with your attacks or she will bust this out on you. Also be weary of using low and mid kicks for she has a really good low and mid kick hold.
If you want me to post more thoughts on how I approach match ups leave a comment, and tell me which character you would want. The character with the highest amount of requests I will do.