Only Energy can levelup heroes.
[1/3] Ways to earn Energy
Every time you come back to the game, you can convert steps to Energy.
You can also open up to 7 step chests from the Fit Tower when you complete step goals. If you have missed collecting step chests, you can click the arrows besides the Fit Tower to go back to previous dates.
[2/3] How to get more Energy
You can boost the step to energy conversion rate with Powerups and Buffs.
[3/3] How to level Powerups and Buffs
You need to acquire Runes. They can be collected by completing battle stages and star rewards. Some buffs can only be activated through the super pass.