Masked Marth

Her refined Falchion gives 4+ to all stats when next to an ally, combined with the atk/def bond seal gives an extra 5 to still and defense. This extra boost can let her eat damage as much as my armored units.

The fury is to give a little bonus to her other stats. The fury plus Falchion enables her to take a round against mages, especially since most mages and even units cannot double her without assistance from skills.

Her b slot skill is the most flexible, with the obvious options of Wraith, Mystic boost, Renewal etc.

Atk smoke 3 in her c skill slot ensures she can take more damage during the enemy phase.

Reciprocal Aid is the choice of assist skill so she can heal up allies, due to her refined Falchion giving her health every second turn. Glimmer is her offensive skill, it will activate on the enemy's turn if she doubles, and ensures the a kill usually.

"A sole warrior stands surrounded by the fallen. Their onslaught never ends, more always come. But still, bleeding and broken she stands firm. An anger in her eyes to scorch the desert sands. Every broken bone, every opened wound, only fuels the fire within. They will pay for their arrogance in blood, their debt collected by the Enigmatic Blade."

This build takes advantage of Lucina's large hp pool to buff her and support her allies. Sealed Falchion and Brazen Atk/Spd 3 allows her to hit pretty hard when she's missing enough health, and even moreso with Vengeance and Wrath 3. Renewal 3 acts as a supplement for the regular Falchion's heal, thus allowing her to still fill the role of a multi-map pseudo-healer with Ardent Sacrifice. Finally, Infantry Pulse 3 lets her reliably shorten ally special cooldowns since she has a nice high hp stat (which also means she'll have plenty of hp to work with when under the thresholds for her skills).

65 Atk/51 Spd unbuffed when in Brazen range. Fury 3 to reduce units that can 1-shot her. Draconic Aura since it's going to trigger during the same combats as Glimmer 99% of the time but kill 99% of the units Glimmer couldn't.
Would recommend Glimmer though if you give her Flashing Blade sacred seal, but I run that on a different unit.

A secondary usage for my Atk/Def Bond 3 seal is to put it on my Masked Marth. If you've read my Sophia build, you know that Bond skills do stack. This includes the refine for the Awakening Falchion, and by stacking that on top of double Atk/Def Bond 3 Marth gains at least a 14/4/14/4 boost to Atk/Spd/Def/Res while next to an ally!

The rest of the set is mostly standard-ish. Ardent Sacrifice and Renewal make Masked Marth an effective Falchion healer while also renewing her bulk every other turn. Hone Atk is used due to Masked Marth working best when next to an ally. Since she'll usually be next to an ally at the start of the turn, Hone Atk will boost up the power of said adjacent ally as well. Glimmer assists in making Masked Marth a powerful burst damage unit.

(Current stats are based on +4 merge.)

She is the mighty MMMMM, or Multi-Map Miracle Masked Marth.

This is a very, very fun version of M!Marth to play, akin to the common Nursina build in function but not focusing on running multiple stacks of HP recovery. She indeed acts as a nurse on multi-map battles like Tempest Trials (benefiting a lot from healing 10 HP at the start of each map) but is a highly functional combat unit, especially if she is adjacent to an ally. Occasionally she can take advantage of multi-map processes by swapping her low HP with the higher HP of an ally at the end of a map, then healing 10 HP and swapping again at the start of the next map.

What really makes her fun though is the wild swings in HP and the momentum in a battle if she engages in combat. She can enter maps with Miracle already charged and at least 11 HP, and that can sometimes be enough to tank any hit; or sometimes she can charge Miracle before what would be a fatal hit thanks to the use of Vantage, and get a kill via a follow-up attack.

The C skill and the seal can be altered without too much changing, I usually forget the use of the C skill and heal via the assist and turn/multi-map processes. This unit was designed before a lot of the newer, more powerful seals were released: this budget Funcina could definitely run a Brazen or a Bond seal.