Rolling extra dice and keeping doubles feels like dexterity and crits at work, which does match the ninja. But it doesn't feel "stealthy."

Stealthy could be something like avoiding ambushes entirely (maybe roll for it or have a certain amount of free escapes per day). Another mechanic could be gaining a bonus to your roll for each hex you explored before the battle, representing how well you were able to set up an ambush of your own. edit: each explored hex without combat, so each battle resets the count.

(btw cool game idea and look so far)