How do combatants upgrade?

Combatants can earn XP in battle. When XP reaches a certain amount, the Combatant levels up automatically. Combatant Max Level depends on their tier. When a Combatant's level reaches the current Max Level, the overflowing XP is restored as XP items. Only after Breakthrough can the Combatant further level up.
You may also use XP items in Growth system to level up Combatants.