Under Advance Growth of the Allies page players will be able to merge duplicate heroes together under the "Merge Allies" tab. Essentially what this does is increase the hero’s SP (if the merged ally is of a lower rank/star of the main hero) or create a sort of limit break which is represented by a + sign next to the heroes level.
Merge Allies Essentials:
1. Both heroes must be the same (same character & same prefix)
2. If the sacrificial hero is of a lower rank/star only SP is raised
3. If the sacrificial hero is equal or of a higher rank/star a + is added next to enhanced hero’s level alongside the SP gain
4. + adds additional stats to enhanced hero
5.If the hero undergoes "Unlock Potential" (raising rank/star) all previously accumulated + and the additional stats given is removed
6. Heroes with + unlocked will require less "Hero Feathers" to Unlock Potential (example: 4 Star heroes require 20, 000 to raise to 5 star, 4 Star +1 will only require 19, 700)