Fir

My +10 Fir build, joined by a Brave Lucina with her drive Breath effect, gets a special cooldown +1 per unit's attack and per foe's attack, charging Aether quickly. The +10 extra damage from the special refine helps with Fir's rather lackluster attack stat and pierces high defenses. After Aether is unleashed, Special Spiral comes in and leaves it at 2 cooldown. The joint drive in the C slot helps increase her attack as well, and the S slot helps in this dual-phase special cooldown. Wrath in the B slot could also work for +20 extra damage.

Very effective build at +10 10. Perfect if Fir has +atk. Build will be very special dependent if Fir is not +10 10. This is more of a player phase build if anything.
Nameless blade eff will act like a better Wo Dao, giving -1 cooldown on special in addition to giving +10 true damage on special.
Reposition is a cheap assist, can be replaced with anything else.
Glacies is there to be the best/main source of damage and with this build, you can rack up to 123 damage (excluding visible and external character buffs).
Mirror impact is there build more bulk to res, build up more damage for special and to give Fir the missing atk that she lacks.
Wrath is very good for Fir since she has high bulk making the likelihood of a OHKO unlikely, this allows Fir to get her ult quicker every start of a turn and racks up more damage to make up for the lack of atk.
Atk smoke is meant so Fir will have a higher likelihood of surviving attacks and make so foes have to spread out.
Brazen atk/res is there to give an extra boost towards her bulkiness at lower hp and give her more atk power to overwhelm foes.
Although this does seem to be an op build, it is highly susceptible to the debuff meta. Make sure to grab heroes who can cleanse debuffs (like restore+ or harsh command+) or to have dancers as an alternative. This build has also been proven to kill max hp/def Arden so make good use of it summoners!

Although Fir faces tough competition from other sword users, she can still carve a niche of her own and become a formidable swordmaster in her own right.

One thing to note that separates Fir from most other sword users is her good base RES stat, allowing her to generally take on mages and dragons better. Distant Counter as her A Slot will allow her to duel mages not named Reinhardt (or some other blue tome mages), while Warding Stance/Breath can help tank more effectively (and allowing her to duel against dragons a bit better).

Fir's worst stat is of course ATK (base 25 is awful), but thanks to her weapon, Nameless Blade and it's weapon refinement (accelerating her Special triggers and increasing the Special damage by 10), this is alleviated somewhat. Iceberg and Glacies work real well thanks to her good RES stat and Nameless Blade reducing the cooldown charge, making them easier to activate (although I prefer using Iceberg because of what I have set for her B and C Slot Skills).

I went with Wrath as my B Slot for the increased Special Damage and making it easier to activate her Specials at 75% HP or less. It combos well with her C Slot - Time's Pulse (thank you Sothis). It combos very well with Wrath (and Infantry Pulse), meaning that at the start of turn 1 and every turn after that (if HP is 75% or less), you will have your Special active (if it is Iceberg). This means that she can potentially one shot a lot of the non-blue units (and even some of the blue units depending on the set).

The Assist and Sacred Seal skills are generally flexible, I just went with Reposition for her Assist (all around useful), and Brazen Atk/Res (for the additional ATK and RES) as her Seal.

ATK or SPD are the preferred boons (I prefer SPD to avoid doubles and potentially double more frequently, but the ATK is a super boon, and the increase in power is always appreciated). Banes would have to go with either HP or DEF.

Fir faces very difficult competition with other sword users, but with time, good skill investment, and proper team support, she can be a formidable swordmaster in her own right.

In my quest to find a unit that could feasibly use the Darting stance seal, I came across Fir and her unique assets, namely, her high res, above average speed, and most importantly, that weapon of hers.
The reason she was chosen over Karel was her speed is higher, and she has stats that are more specialized.

The idea of this build is similar the "Remington" that units like Laslow and Hinata can run, but works fundamentally different, leading me to dub this style of build a "Reverse Remington".

The basic idea is Fir utilizes the Darting stance seal and Threaten C skill to gain +11 speed advantage to have effectively 50, doubling nearly anything that attacks her with natural speed, then retaliating with a nasty +20 Glacies coming off her 31 (35 with breath) resistance, making a pretty surprising nuke.

Her lower defense somewhat works into her favor here too, as she has very little chance to be doubled naturally and the enemy needs to OHKO her to avoid the nasty Glacies, or run guard.

As with any enemy phase build Firesweep is the bane of this set's existence.

Tl;dr, Res make enemy phase a big boom boom because fuck dragons I guess

This set turns Fir into a cheaper Ryoma (minus the lack of Distant Counter).

With Fury 4, (Neutral) Fir can reach 46 Atk, 40 Spd, 28 Def and 35 Res and with 8 damage recoil from Fury 4, Fir can easily reach the threshold for both Vantage and Brazen Atk/Res in just one combat, pushing her stats to 53 Atk, 40 Spd, 28 Def and 42 Res. This means she can proc Glacies to boost her damage by 36 with an additional 10 damage due to the Nameless Blade and combined with 53 Atk, that's pretty much a death sentence to a lot of units and with Vantage, Fir will always get the first strike which makes her a good unit to bait out melee units, especially Dragons due to her high Res.

Def Ploy is the best choice for Fir's C skill as due to her high res, Fir is able to inflict Def Ploy on most units which helps to boost Fir's damage output.

If you're concerned about Fir's longevity, you can swap her seal for Brazen Atk/Def which allows her to survive against melee units better but would reduce her effectiveness to deal with Dragons.

The best boon-bane set for Fir is +Atk and -HP, -HP is ideal as it allows Fir to easily reach the threshold for Vantage and Brazen Atk/Res and due to Fir's low Atk, an Atk boon will greatly boost Fir's damage output in the long run.