Spring Kagero

As a high attack dagger unit, who obviously has high speed, it is best to take advantage of both, but, to ensure that she doubles, take a flying unit with Performing Azura with her, so that she gets +6 Atk/Spd at the start of every turn, granted that they're adjacent to each other.
The reasom Barb Shuriken is used, is so that, when within Desperation range, you are guaranteed a hit with glimmer every turn, assuming that you have the speed to double, or that your opponent doesn't have Wary fighter.
One other thing to make sure on, is that she isn't within the danger area at the end of the turn, as, if any opponent launches on her, she will most likely fall to one hit, but there is a chance that she won't, depending on the opponent, but you need to make sure that you have a couple of units with reposition or any other movement assists, to keep her safe.
Also, you could use the Speed Smoke Seal to make it a quad smoke build, rather than Speed +3, if you feel more adventurous.

Death from Above

Kagero's always been a strong dagger unit and, much like Hector, her seasonal variant is no exception. Being a flying unit, this allows her to enjoy the buffs received by Hone and Fortify Fliers. In a sense Spring Kagero can be seen as a psuedo-Blade Tome unit, just without the whole "more buffs = more power."

Her default weapon is like a watered-down version of Peshkatz, but it gains a Wo Dao+ effect which is pretty good for strong hitters like her. Other options include her old Poison Dagger+ or the Kitty Paddle+ to deal with infantry and mages respectively. All three should have a Speed refine to make sure she can be as fast as possible.

Moonbow is her special which can be triggered quite often and seriously bring the pain to even armored units. Glimmer is another option if straight up power is more desired.

A-skill is Fury 3, which adds to her power and speed. While the recoil may seem like a detriment, it helps her get into Desperation range which ensures she can get a clean kill before her enemy can retaliate.

B-skill is Desperation 3 which will allow for immediate follow-ups and with Kagero's power, that means doom for whoever is facing her.

C-skill is Fortify Fliers which will really come in handy for the Blade Tome fliers like Nowi and Morgan, though her natural Goad Fliers works as well.

The seal is the most flexible of her set. Heavy Blade allows for faster charges, though if paired with Desperation and either Moonbow or Glimmer, the second shot is always going to fire off the special. Attack+3 and Speed+3 aren't bad either if you want some more hitting power or more speed to ensure doubles.

+ATK -DEF

This build takes advantage of Lethal Carrot's +10 increase in special damage and makes AoE specials incredibly powerful. With Heavy Blade activating (54 attack unboosted while initiating is really good, and with flier buffs its better), your AoE special will be fully charged for her next battle. If AoE isnt your thing, you could go for Moonbow instead because it procs every fight with Heavy Blade and assuming you are in Desperation range. I think this build is very fun to use and helps a lot with patching up her lowered arena score due to the def superbane since AoE specials increase score more than regular specials.

Reposition is a great assist skill for fliers, but if you want more score, a dual rally skill works too.

Swift Sparrow 2 is a fantastic option for her, provided you have the fodder for it. If you dont, which is understandable, Fury 3 will also get the job done, as it would open up the possibility of running Ploys on the C slot, SS slot, or both, as 30 res isnt too bad for Ploying.

The C slot depends on your team. On a non-flier team, Threaten Def works great for increasing her damage output, but if you don't have threaten def, Threaten Spd is a good alternative so she can double some of the faster units. Ploys can work too, although its better with Fury instead of Swift Sparrow, as with Fury your res will go up to 30, as stated earlier. In flier teams, Goad Fliers is fine as is.

Heavy Blade is great if your Kagero is +ATK like mine. It charges up Growing Wind to full after every battle and allows Moonbow to proc for every battle, assuming your Atk is higher, which it will for a big chunk of her matchups since 54 atk is really good, and can be pushed to 60 with hone fliers buffs. If you don't have +ATK, or do not feel like using Heavy Blade, +Speed 3 works great too, as well as one of the Ploy seals or Smoke seals for more debuffing.

Enjoy your Ninjabbit!

(Stats are from my own Kagero, +Atk, -Def, Weapon and Skills included) In the New Spring debuts Spring Kagero, the first and only Flying Dagger user in the entire history of Fire Emblem! Not only that, but she also debuts as the first ever Colorless Flier introduced into Heroes. Being a unique class that nobody expected to appear, This Ninjabbit is more than just looks.
1. 35 Atk and 34 Spd, both at neutral let's her tie with Sothe for the best Offensive Stat Spread for dagger users. Howevere Kagero trades off Sothe's utility thanks to his Pechkatz for the ability to be a reliable mage baiter if enough investment is made into her salvageable 27 Res. This allows here to be quite flexible with her IVs and builds, she can be built to be an all out offensive unit, a Mage bait, or Support-Damage Hybrid.
2. Being a Flier grants her 2 things: access to the Hone/Fortify Fliers buffs and Guidance, and unhindered mobility. Hone Fliers makes her already scary offensive stats downright terrifying, while Fortify Fliers turns her into the best mage tank of the flier class, as she does not struggle against any color of mages, F!Morgan can sometimes struggle to finish off Green and Blue mages with high Resistance while Spring Kagero does not and she can always run Cancel Affinity to fight TA+Raven tome mages. Guidance gives her more utility in Mixed Movement teams, helping Infantry and Arnored allies move around. As for the mobility part, Kagero so far is the only non-infantry dagger user so far, so she has the best mobility out of all of them.
3. The Debuffs from daggers are not to be underestimated. They can severly cripple the opposing team if used right. When teamed up with a dancer, Kagero herself can use the debuffs to deal huge damage to even Defensive units such as Lukas, Eldigan, and the armor units. She can also apply her debuffs much safer than other daggers since she can apply them over unpassable terrain without risking getting counterattacked in the Enemy Phase.
Whether be for a Flier Emblem team or a regular one, Spring Kagero surely has proven to be a worthy candidate for either one. Her stat spread and Flier mobilty allow her to be one of the best Dagger users in the game, whether it be as a nuke, a Mage Killer, or just as a Debuffing-Support unit.