I cant really think of a good way to use kestrel stance other than with vantage. Even then, swift sparrow/desperation would be a better combo since it doesn't allow the enemy to attack. Also seeing that Ryoma is quite fragile in both defenses it doesn't seem like he will ever put it to good use.
Swift Sparrow and Desperation is a Player Phase combo. Vantage is for Enemy Phase.
The thing is that they gave Ryoma enemy-phase skills, but everyone's looking at him as a player-phase unit, so it's kinda hard to see just how he can make use of it. He can actually have some fairly good defense if you give him a Fortify Fliers buff, along with some Ward Fliers.
Kestrel Stance would only work effectively for some units since it buffs attack and speed rather than defense.
Does it really make sense to run tanky fliers? If the "impenetrable wall" is the strategy then armours do it better because they have significantly stronger defenses and also have class buffs. Plus the only arrow they're ever scared of is the Warrior Princess, though argument could be made that they have their own worries in Wing Sword and Thani and etc. which...never have 5 range, at least.
And even as tanky fliers go, why not L.Robin or DC Myrrh? Both have adaptive damage and reliable ways to avoid getting doubled while doubling back with QR. L.Robin can even stack bracing stance with Iote's Shield.
1. Not everyone likes Armor Emblem. Flier Emblem has the ability to fly over mountains, forests, and rivers (and a few other terrain problems in PvE content) and don't have to worry about having to have Armor March to move two spaces.
2. Not everyone has L!Robin or Myrrh, let alone Distant Counter to put on her.
For example, even if there are skills that perform better than Kestrel units that use it well are Berkut's Lance units and Safeguard units since you get the +7(?) to Def or Res along with +4 Atk/Spd on enemy phase. As long as the unit can sustain themselves or has Vantage they could have an enemy phase build ready for use.
I'm planning to build my +def/-HP L!Ryoma using his base kit (perhaps swapping his C slot) and give him Iote's Shield to be the primary bow counter for one of my flier emblem teams. The other team will get FGrima. Kestrel Stance can work on its own or with Vantage, much like how a Zelgius with Fierce Stance/Warp Powder would function. (I mean, having Vantage on them is mightily fun but not having it does not render them obsolete.) It may only work on a +def Ryoma because his defense is borderline (28) or with Fortify Fliers access, but yes Ryoma can be a good enemy phase unit if built as such.
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