Dataminers have found references to what seems to be a new support system that is heading to Fire Emblem Heroes in the near future.
Just like how the support system works in the traditional line of Fire Emblem titles, when two allied units are close to each other on the map, they will gain a stat boost.
The full description on how it works can be found below.
MID_TIPS_0018: Strengthen your allies through Ally Support! Allies who are partnered up will gain a stat boost when they fight near each other.
You can find Ally Support under Advanced Growth in the Allies menu.