From Fire Emblem: The Blazing Sword, Ninian was one of two dancers from the game and is (probably) Roy's mom, but she never really fought... until now anyway.
Faithful Breath triggers a Def/Res Cantrip effect whenever she uses her Prf Dance assist, but it also enables Ninian to fight: if her HP is over 40%, she gets a power boost, and if she is initiating combat and has a Triangle Advantage (read: Blue) or if a [Penalty] is active on her target, they can't counterattack without the Null C-Disrupt skill. Dragon's Dance is just like any other dance normally, but when it's been used one turn after the battle has begun, Ninian (and her pair-up cohort) will also be given another action and an Atk/Spd bonus, as well as the Isolation debuff. While this needs a few turns to recharge, only one unit needs to be out of place for her to charge in and end the battle on the spot...
She also comes with the Atk/Spd Catch 4, Spd/Res Near Trace, and B Feud passives, and Dragon Fang will scale off 50% of Ninian's Atk. On Abyssal, she is near-guaranteed to have a penalty inflicted if an enemy is left in a cardinal direction of her thanks to the Panic Ploy seal.
The Oracle was felled once before by Durandal. No version of it here may be able to end her as fast as it once did, but there's more than one way to slay a dragon...