My friend uses Ky as his main, and I tend to have the most trouble fighting him, (he's played against me so much, he knows all my combos like the back of his hand, and does a great job blocking me).
Main things I go for,
1. Watch for stun edges and punish with puffball. Tends to lead to a half health combo.
2. Stun dippers are ridicously easy to parry. If the Ky sends one out and hasn't comboed you into it, you can block or even get hit by the first hit (part where Ky slides forward), and parry the sword slash followup. The parry leads to a nice s -> hs (3 hits) -> qcf s + k -> ryujin (carded) -> qcf s + d a few times -> qcf s + hs -> one corner loop iteration. I usually net a dizzy into instant kill in this situation. Also greed saber is pretty easy to parry as well, and it leads to into air to corner loop situation.
3. Most Ky players I know enjoy the air, which gets high damage anti air combos.
4. Becareful using puffballs, stun dippers beat them like crazy.