Masked Marth

This is my budget build for Masked Marth! I use her mostly in Tempest Trials as she works great as a buffer and healer.

M!Marth has the same IVs as a neutral Lucina. That said, Darting Blow 3 is chosen to take advantage of her above average speed. When initiating she is able to double hit her enemies with 43 speed, activate Luna quite quickly and survive many encounters, as other units may not be able to double her.
Renewal 3 is chosen for 20 HP healing every 2 turns, which works great with Ardent Sacrifice.
Double Breath of Life 3 is chosen to have her healing even more every time she initiates attack with an ally by her side.
Double Breath of Life works great along her Falchion refine, as she has to be adjacent to an ally for both passives to take effect.

A build to fit the dire resolve that we have in this desperado. Some of the skill choices may not be 100% combat-optimal.

When in desperation range, Sealed Falchion and brazen atk/spd combine for a mighty 62 atk 48 spd before merges or buffs. With galeforce, she will fight, and fight, and fight with every last bit of hope and strength she has left.

If retreat is the only option after taking the first opponent, reciprocal aid can bring her back out of desperation range for another attempt at a one-turn galeforce.

At the time of writing, it seems that this build is the only galeforce build offered for Masked Marth.

One of the best Tempest Trial rewards in my opinion. Marth is basically a budget Lucina with no access to IV's but is easy to merge given the amount of TTs that she's been the reward for. She is a very versatile unit, with the potential to become an offensive sweeper, dragon slayer, combat medic, and defensive tank given the respective build.
This build turns Marth into an independent, offensive sweeper with Sealed Falchion. The idea is that Marth will engage or be engaged into a round of combat, which should be enough to activate both Sealed Falchion and Brazen Atk/Spd 3. This grants her +12/12/5/5, which is insane, and can be made even better with Hones, Fortifies, Rallies, Tactics, etc. The speed boost allows Marth to rival even the likes of speed demons like Ayra, and Karla.

Masked Marth is currently merged +2, but that doesnt matter for his use. Weapon refine also does not matter.

I use him as a Tempest Trials Healer unit. Because of his Renewal 3 B Slot and the Renewal 2/3 (depending on weapon) he heals nearly every turn, and if one of your units is low you just Reciprocal Aid them.

Darting Blow raises his attack so he can also be used offensively. Hone Atk 3 and Spur Spd 3 help buff allies.

This build utilizes Lucina’s standard Budget Build but with more survivability. Due to Fury 3’s -6HP recoil, Masked Marth’s HP is sure to dwindle down more quickly than desired.

While the Renewal 3 attribute of the Falchion heals +10HP every 2 turns, it is still not enough to help fill her massive HP pool. Reciprocal Aid also helps refill her health by trading her allies’ HP with her own, but this is also where Aether can come in handy, if you have extra Aether fodder lying around. Because she deals plenty of damage and is speedy without the aid of buffs, she can charge Aether reliably. Luna can also be used for the same damage output but a shorter charge.

Reposition is also a recommended Assist as it helps Masked Marth move her allies out of harms way and trigger Desperation 3 after combat. With the added effect of the refined Awakening Falchion, Masked Marth should be able to survive one, even two rounds, of combat with an ally near her.

Desperation 3 is preferred so Masked Marth normally has attack priority on Player Phase to eliminate enemies and charge/activate Aether, but Vantage units can very easily ruin the battle.

The C-Skill Slot is flexible, but Threaten Defense 3 is preferred. It’s self-explanatory as it lowers surrounding enemies’ Defense by -7, but also requires Masked Marth to survive a round/several rounds of combat even after proper positioning.

Infantry Pulse is another preferred skill if Masked Marth is on the same team as another Infantry unit or on a full Infantry team. It helps utilize Masked Marth’s large HP pool when used in conjunction with a refined Awakening Falchion.

Preferred Seals would be Flashing Blade 3 to complement her high speed and to help charge Luna/Aether or Threaten Speed 3 to assist in ensuring a Desperation 3 attack.