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.
The max amount of stamina increases as Ninja Rank gets higher.
A stamina is recovered by every 3 minutes.
Stamina is restored to full when Ninja Rank increases.
If players do not want to wait, they can spend Ninja Pearl to refill stamina.
