Brave Lucina

Another Lucina to make the Steroids Lucina even buffer. She gives +6 atk/spd to all team members within two spaces. On top of that she can also fend for herself with the rest of her skills. Oh btw, she's +1.

Boon/Bane: +Def/-Res
Weapon: Slaying Lance+ (+Spd) / Berkut's Lance+ (+Res)
Support: Reciprocal Aid
Special: Aether
Passive A: Steady Breath
Passive B: Wrath 3 / Lancebreaker 3 / Daggerbreaker 3 (not with Berkut's Lance) / Guard 3
Passive C: Drive Spd 2 / Infantry Pulse 3 / Infantry Rush 3 / Any Tactic skill
Seal: Panic Ploy 3 / Close Def 3 / Quick Riposte 3 / Quickened Pulse (if using Berkut's Lance)
Summoner Support: Absolutely
Ally Support: Sword or Red Mage for protecting her against Green units, or Axe or Green Mage so she can protect them. Someone that can counter enemies like Hector would be great (Roy is the best unit to do so with, due to his innate Triangle Adept and Binding Blade — both unrefined and refined). If using the Panic Ploy seal, it would prove valuable to have another heavy-hitter that can deal with Emblem teams alongside her (like VL!Ike).

Super-sustain pseudo-Healer Lance!Lucina. With Slaying Lance+ (+Spd), she can get up to 39 Spd (42 if you have a Spd boon instead), and due to the combination of it and her speed with Steady Breath, she can fully charge and activate a 5-cooldown skill like Aether in a single battle. Due to her HP being higher than 40 aside from an HP bane (and that would just take away from Aether...), Reciprocal Aid is a better option for activating Wrath. Obviously, her Special is Aether which is amazing at both damage and sustain. Activating this each time she battles an enemy is pretty brutal.
Obviously again, her A skill is Steady Breath. While this does force her niche into becoming an Enemy Phase unit, a Def boon lets her reach up to 35 Def with SB's effect...and with a neutral base 34 Atk (48 with the weapon) she'll be retaliating pretty hard after your opponent does next to nothing. Unfortunately, this Def increase does nothing against Green or Res-targeting units, so keep her away from them unless she's initiating a winning fight.
For her B skill, either Wrath 3 or Lancebreaker 3 are your best placements, or Guard 3 for more safety. Wrath 3 lets her charge Aether even faster and increases its damage by 10 (and therefore potential healing by 5), while Lancebreaker 3 gives her some safety against Lance-using units that can deal some nice damage to her. Daggerbreaker 3 will be useful if she ever faces a Felicia using her Plate on the Enemy Phase, but due to her low Res she'll likely fall to her regardless unless equipped with Berkut's Lance+.
For her C skill, either her natural Drive Spd 2 or an inherited Infantry Pulse will work. A Tactic skill like Res Tactic or Def Tactic is also incredibly useful for mixed teams, though they are incredibly rare skills from powerful units that should not be sacrificed so easily. Same with Infantry Rush, which is a fairly new skill that applies a Heavy Blade effect on allies in Drive range.
As for her seal, Panic Ploy 3 is my favorite to use against Emblem teams. For a bit more Def/Res against her counterable enemies, though, Close Def 3 is equally if not more usable. If you worry about being able to counterattack to get Aether activated due to a skill like Lancebreaker or Wary Fighter getting in the way, Quick Riposte 3 is your best friend as it negates the latter and somewhat negates the former (in fact, due to Steady Breath's effect it may be better that Lancebreaker still doubles so you can get an extra charge on the next usage of Aether).
For the alternate build with Berkut's Lance+ (+Res), you'll need to switch out your Seal with Quickened Pulse in order to make Aether's charge start at 4. While it's slightly slower than Steady Lance, Berkut's Lance is much safer when having to deal with mages and/or dragons...and even with a Res bane, having Berkut's Lance+ with the Res refinement will patch it up enough.
For your Ally Support, be sure to take an ally that can either help or be helped from Lance!Lucina's status as a lance unit. A strong unit that has lower HP than she does will be able to be restored to full health from her Reciprocal Aid depending on her current HP at the time, and she can make use of Steady Breath/Aether to get back up to full health for another pseudo-heal. A unit that can stand alongside her on the front lines, preferably one with a Distant Counter weapon/A like VL!Ike, is also perfect for her (it also helps that he's a sword user with DC built into his weapon, so he can protect her against axe users like Arthur AND Tome users like Nino thanks to his natural Warding Breath).

Drive skills are so awesome. They can't be turned into debuffs, which makes them fantastic for arena play.

Lucina's native weapon is fantastic. It buffs her modest defense and makes it rather tanky. But the support ability is what is key here. It naturally provides Drive Attack and Drive Speed to ant physical units (brave units love her). If you add on Drive Speed C-slot and the Drive Attack seal, you double it and she can also buff magic attackers. +6 Atk/Spd is amazing. It'll keep your slow units from being doubled and allow your modest speed units to demolish attackers.

I let her keep Aether for healing (if doing TT or chain challenges) or for finishing off tough enemies in the arena. Her speed is decent enough that she doesn't need Lance Breaker, and Vantage isn't what it used to be. Wrath is perfect for her, helping with the Aether charge.

Brave Lucina has such a fantastic offensive stat spread, that it would be a waste not to use it. This build will take a more offensive approach, rather than a supporting approach.

Spd will be boon since what this build will be all about will depend on her doubling ability, and Res will be bane since it doesn't really help her in the long run with what she's doing.

Harmonic Lance+ is chosen here because this bolsters her offensive potential, and with 34/36 offenses, she can be seen as a lance-wielding Hana. This is further refined to give her the edge she needs to proc her specials more, so speed is mandatory on the refine.

Swift Sparrow 2 is chosen here over Life And Death 3, because Brave Lucina is a rather decent duelist, so compromising her bulk wouldn't necessarily be ideal even if it comes with bolstering her offenses more. With this, and Speed+3 seal, she reaches 52/49 offenses on the Player Phase, unboosted. A budget option would be Fury 3 as it gives nearly similar offensive boosts while also boosting her bulk, but at the expense of bleeding damage every combat. This is also handy because this can help her reach the threshold of her selected passives below.

Desperation 3 is chosen both offensively and defensively, as this not only potentially lets her get ORKOs before a retaliation, but also potentially saves her from a retaliation that could have KOed her. This comes especially handy under circumstances when she has a special on standby. Drag Back or Hit And Run can also be used when you need a retreating option, which works favorably with repositioning or dancing team mates.

Drive Spd 2 is her default C passive and it's generally a great option all around, but if you want other options, then Threaten Def 3 can also be used if you want more damage from Brave Lucina.

Reposition is chosen as her assist because it's generally an all around great assist. Moonbow is chosen as her special, because its low CD helps her proc extra damage from Harmonic Lance+'s effects.

Sacred Seal can be Speed+3 to help Brave Lucina double more targets and charge her specials more. Brash Assault 3 can also be considered if your Brave Lucina is doubling more than enough enemies and you have a reliable way of getting to the 50% threshold. Brash Assault 3 can help her take on enemies that are otherwise problematic, such as Wary Fighter armors like Zephiel. Though, do note, that if running Brash Assault 3 seal, you should be running Desperation 3 in the B passive slot.

This is an unconventional build for Brave Lucina compared to her Geirskogul support builds, but it will prove to be effective if given the chance.

This build turns the very supporter herself, B!Lucy, into an offensive/defensive god with the right team suppport. (If you run a full Infantry team, and have a teammate with over 43 hp, use Infantry Pulse on that ally, and swap Quickened Pulse for Close Def 3, or whetever you'd like) I'll explain what this set is about and will do my best to break the meta even harder :), ok, let's dive into it then. Replacing Geirskogul will rob her from her supportive job, but with Slaying Lance (+Spd), she can double enemy's with 52 spd (56 if fully merged), and lemme tell you, that's quick. The Dual Rally is here to help procc Spd Feint, which lets her double 52-56ish spd enemy's back when needed. Atk/Spd Bond is heavily team oriented, but since my team is literally 3 Dual Rally's and a Dancer in the mix, she'll be alwasy with someone, who'll boost her up spd. Spd Feint (also known as Spd- Bridal Tharja) helps her to double many fast reds, blues and literally everyone who has high spd. The C skill is your choice to make, but I prefer Drives to support my team. Quickened Pulse helps the Slaying Lance+ Aether combo to work, in which she becomes a high treat to all of your Ayra's out there. (you can add any Seal but Quickened Pulse if you have an Infantry Pulse user in your team). In summary, this build removes her supportability, and adds offensive power to her. GG no re nerds rawr xd