(A big thanks to Nintendo Everything for the pictures)
Firstly, Azura’s mysterious songstress costume is unlocked when you get her. In addition to wielding lances, she can use Sing to restore the Warrior and Awakening gauge of allies. The other four costumes are unlocked by playing the new history maps.
Speaking of, the three history maps are loosely based off the Hoshidan capital, the Ice Village and Izumo. One of the new rewards is the “Bond Talisman”, which can be equipped in place of a healing item and increases the rate of gaining support points.
In terms of the new characters, Oboro has the Counter skill, which opens up a stun gauge when guarding an enemy attack. Meanwhile Niles has the Lethality skill, which increases the power of critical hits proportional to the user’s Luck.