Nariko
Try to perform consecutive hits on enemies to build the combo counter on the left side of the gauge up as much as possible. If she got injured or if she haven't made a successful hit for a few seconds, the number will reset.

Kai
You must score consecutive hits as well, though building up the gauge is made considerably easier because her time constraints between hits are much more lax, and her use of aftertouch (when shooting arrows) allows you to guide the arrow to its intended target.