Nintendo has opened a Japanese page detailing the functionality of the Alm and Celica amiibo in Fire Emblem Echoes: Shadows of Valentia. Right out of the gate they will unlock a special dungeon that can be used to gain experience, collecting items, and developing characters. They’ll also allow you to summon phantom versions of Alm and Celica in battle that have the same stats as the data written on the amiibo.
Phantom soldiers can be summoned by scanning other amiibo as well, but they won’t have the ability to save their stats.
Fire Emblem Echoes: Shadows of Valentia launches for Nintendo 3DS on May 19th, 2017.