Stamina is used to enter battles. Please note that some events or battles don't require Stamina. Stamina regenerates by 1 every 5 minutes, and fully refills when you leveled up. The maximum amount of Stamina you can have increases as you level up.
Stamina can be earned through:
• Login reward
• Mission reward
• Underground battle reward
• Event reward
• Friend Gacha
• Support Packs
• Lan Fei's Shop (purchase with Crystal)