Mia

Preferred IV: +SPD / -RES
Weapon: Armorsmasher+ (EFF)
Assist: Swap / Reposition / Pivot
Special: Aether / Moonbow / Galeforce / Luna
Passive A: Fury 3 / Fury 4 / Swift Sparrow 2 / Flashing Blade 3
Passive B: Null-Follow up 3
Passive C: Even Def Wave 3 / Even Res Wave 3
Sacred Seal: Flashing Blade 3 / Atk/Def Bond 3 / Close Defense 3

+spd/-res Feel free to go with another bane if you so choose, but after a merge it no longer matters. The +spd is rather important for any Null Follow-Up builds and with some armors being deceptively speedy, you willl want to capitalize on Mia's speed.

Resolute Blade really hurt to switch to Armorsmasher. Those 5 points of attack were sacrificed because it just was not killing armors enough (specifically Surtr and Legendary Tiki), even with the Wo Dao effect. When the meta shifts away from armors, you can change back to the more universal weapon.

Aether (or even Galeforce for you sly dogs out there) for arena, Moonbow or Luna for every where else. Aether and Moonbow are preferred because it will help in completely obliterating defenses, which is the name of the game for this build.

Fury 3 or 4 works well because Mia need all the stats she can get to overthrow our armor overlords. You are more than welcome to keep Flashing Blade on A and put an Atk/Def Bond 3 in the seal. However, that drops her RES and Mia tends to run out on her own, so having stats in Fury that she can take with her is hard to replace.

Null Follow-Up is obviously the key to the whole thing. A lot of people forget it has 2 effects. Not only does it stop Quick Riposte, Bold Fighter, and Vengeful Fighter, it also negates Wary Fighter, Great Flame, and the entirety of Swordbreaker. This leaves it to a 100% pure speed check which is why I gave it to Mia as very few people can check her speed. Throw in her very serviceable defenses and she is one of the top people for the skill in my opinion.

I feel that the Even Wave skills are great for your defenses and enemy phase moments because by turn 2, the enemy has arrived and you can adequately be buffed to take their hits. By turn 3, you are cleaning up with your player phase again. If you feel like you are good on Defense, then Even Res Wave is a good switch.

As is the case with any unit, so long as you reasonably build your team around her, Null Follow-Up Mia can take down the current meta easily.

My Mia is built on Speed. She will be doubling units a lot, and focuses on lowering enemy speeds for her benefit, and her teammates' benefit. Due to outspeeding almost every unit in the game, she will charge her special very often, and Astra will be ready to go in 2 hits. If she's under 75% HP on Enemy Phase, she can activate Astra and finish the enemy off without taking more damage.

since i got an -atk mia, ans i have no need to fodder for her, so i decided to make a tanky meme.
safeguard could be switched to barrier blade, if you want it to be a anti dragon unit.
support skill is very flexible
bonfire for special so it can be paired with flashing blade and the high 44 def in enemy phase, you could have breath skills instead if you have fodder.
swordbreaker to counter top tier sword units in arena(zelgius, ayra, black knight.etc)
c skill could be flexible, i use threaten def or res skills for bonus unit to clean up if mia doesnt kill.
s-slot is c.def, the other most crucial part aside from the safeguard/barrier blade
for synergies, good ranged green units are welcome(lewyn,filer nino,bride sanaki,new year fjorm)
good support units(male or female robin with their refined prf tomes, double tactic units,brave lucina)
battle medic(eir )

Used for Arena Defense, Distant Counter and Wrath make sure she can retaliate damage from every distance, Spd Wave boost her speed to 49 on Odd turns, making sure she doubles everytime it activates and uses Flashing Blade to activate Luna in every battle on EP, debuffs allowing.

Merge: +4
Seal: Heavy Blade 3
Blessing: Earth

Resolute Blade (Weapon)
Effectively a 19MT Wo Dao. Mia has pretty above average Attack, but appreciates any Attack boost she can get. Her sheer power and blazing speed makes her a deadly user of this weapon, especially if she's armed with Moonbow or Glimmer.

Reposition (Assist)
Very self-explanatory.

Moonbow / Glimmer (Special)
Mia can effectively run one of the two Specials depending on the situation. Moonbow is her go-to Special because it allows her to reduce the bulk of her enemies while Glimmer outright destroys physically fragile units, including some Blue units.

Death Blow 3 / Fury 3 (A-Slot)
Primarily used to trigger Heavy Blade easier. But it also gives Mia a much needed boost in Attack.She is, however, not weak by any means. She can alternatively run Fury to boost her offenses and defenses, whilst taking chip damage from it to help her get into Desperation effect.

Desperation 3 (B-Slot)
With Mia's incredible Speed, she is a very solid user of Desperation. It also synergizes well with Fury.

Drive Atk 3 (C-Slot)
Normally Mia is not much for support, but if need be, she can be. Though, she would prefer skills that benefit herself most of the time.

Heavy Blade 3 / Flashing Blade 3 / Brazen Atk/Res 3 (Sacred Seal)
Both Heavy Blade and Flashing Blade work well with Mia and helps her activate Moonbow/Glimmer every round of combat. However, in the scenario where both seals are taken, then she may resort to use Brazen Atk/Res instead.

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Pros:
+ Excellent offensive stats of 32 Attack and 40 Speed
+ Comes with a powerful weapon, Resolute Blade, that is basically a 19MT Wo Dao
+ Natively comes with Flashing Blade skill, which can help her activate Specials easier, but prefers it in a form of a Sacred Seal
+ Mostly very flexible to build

Cons:
- Mia's base kit lacks synergy in general and requires a bit of investment in order to become a very competent unit
- Has competition with Ayra and Karla, the two who have skills that involve with Speed, however she loses in majority of the matchups and is better compared with Soleil instead
- Naturally weak to Swordbreaker and vulnerable to majority of tanky Blue units