After clearing Stage 2-15 you will be able to sync heroes at the Academy.
Syncing your heroes allows you to gain the effects of levelling up a hero without using any resources. When you choose which hero you want to sync, that hero's level will raise to the lowest level of your top 5 heroes.
Your top 5 heroes will be displayed at the top of the sync page.
A synced hero's level on their icon will appear as neon blue.
Once a hero is synced, you will not be able to level that hero up individually. The only way to increase the level of a synced hero is to level up your top 5 heroes. The synced hero is not synced to the hero they originally synced with, they are synced to the the fifth highest level, no matter which hero has the fifth highest level.
When you first unlock the Academy, you have 3 sync slots. To unlock more, you will need to use Sync Points or Diamonds. You can get Sync Points from Idle Rewards.
You can have as many heroes synced as you have available sync slots. All will be synced to the lowest level among the top five heroes.
When you no longer want your hero to be synced, you can desync them. Desyncing a hero resets their level back to the level they were before they were synced.
After a hero is desynced, the sync slot they occupied has a 24-hour cooldown period. That sync slot cannot be used by another hero until the cooldown period is over.