Missions require players to spend the certain amount of stamina. The amount of required stamina increases as missions become harder. Also, emergency missions require more stamina than that of story missions.
Stamina and Ninja Rank
The max amount of stamina increases as Ninja Rank gets higher.
How to Refill Stamina
Recovering over time
A stamina is recovered by every 3 minutes.
Increasing Ninja Rank
Stamina is restored to full when Ninja Rank increases.
Using Ninja Pearl
If players do not want to wait, they can spend Ninja Pearl to refill stamina.