【Updated: 】
You cannot send allies home or use them in Merge Allies in the following situations:
・ If your Hero has been marked as a favorite.
・ If your Hero is in a reserve barracks.
・ If you have selected Alfonse: Prince of Askr, Anna: Commander, or Sharena: Princess of Askr.
・ If the rarity of the source ally is lower than the rarity of the target ally.
Note: You can merge your allies in this case by enabling the Merge Heroes w/Lower ★ Rating option in the Settings menu.