Upgrading

Each hero has three main forms of upgrading their power, levels, rank, and grade.
 * Levels are gained using gold that you collect from stages, but resets after ascending.
 * Rank is increased with Spirit stones that are gained from ascending.
 * Grade is gained from hero scrolls that can be purchased from the shop and arena store.

Levels
Without any upgrades, all heroes cost to increase their level is the same. The leveling system can be broken up into 10s, Each time you level your unit the gold price increases by a small amount, then at the 10th level it increases the price by 400% then repeats the cycle for another 10 levels.

Levels can be increased indefinably but reset at ascension.

Rank
Rank can be upgraded indefinitely(this may have a cap but have not reached it as of yet), each rank increases DPS by roughly 30%

Grade
Grade can be upgraded up to a max of 5, each time you increase your grade increased it by 400% of the previous grade. Grade can only be increased by obtaining hero scrolls, which are obtained from the shop and arena store. All heroes' prices for grade are the same.