Sophia

Due to her Personal Weapon having Bracing Stance, her potential in Enemy Phase should be focused even more than Player Phase.
With Skills like Steady Posture and Swift Stance, her insanely low Spd rises up by 10, reducing the number of foes doubling her by a lot, due to Lull Atk/Spd giving her effectively up to +13 Spd when attacked.
With Weapon and all Passives combined, also Rouse Def/Res , her Statgrowth on Enemy Phase lies at a whopping +13Spd/+19Def/+17Res wich ends her up having 34Spd/52Def/48Res without considering a Merge or even Dragon-Flowers
With a Full +10/+10 Build, she ends up at 53/57/42/60/56 if attacked, making her a Wall to be reckoned with

First merge project, I came up with the idea after messing around on the Unit Builder. Gonna give her Fury 4 if I get Griel, probably won't, as I've gotten 4 Mists now..

Anyways, I haven't seen any good Sophias and I like her character design, so I decided to build her.

Enemy Phase Close Counter Sophia build with an emphasis on tanking hits and retaliating back with her special.

Key components are Guard, Quick Riposte and Fierce Stance. Due to CC taking up her A-slot you will be forced to choose 2 out of these 3 skills. Guard will help make Sophia tankier by denying enemy specials, Quick Riposte will allow her to use her own special either in a single round of combat or right after depending on whether you are using a 3-turn or 4-turn special while Fierce Stance compliments her enemy phase play style well and works best with an ATK boon.

It's also worth noting that due to her legendary weapon being a raven tome, Sophia is a solid user of Null-C Disrupt to eliminate pesky dazzling staff/colorless sweep users if you are willing to invest that much into her.

Sophia's in a weird niche like Faye, with 28/29 defenses and a decent 40 HP. This build is built around mixing both of Sophia's defensive stats, with emphasis on Resistance and team support, allowing her to become a powerful mixed wall..

A +RES boon allows Sophia to maximize her Resistance stat, allowing her to tank attacks from mages more easily while also ensuring stronger activations of Iceberg; however, if +RES is unavailable, then +ATK works just as fine, as it improves Sophia's overall damage output. A -SPD bane is appreciated to not lower any of her other stats simply due to the fact that it's her worst stat and Sophia uses her other stats much more often.

In case you didn't know, the effects of Bond skills stack. Using both Atk/Def Bond 3 and Atk/Res Bond 3 in the Sacred Seal and A Slot respectively grants Sophia +10 Attack and +5 Defense/Resistance while next to an ally, which sets her optimal stats to around 43/56/16/33/37 (excluding merges). Eternal Tome (+Eff) also grants Sophia the effect of Bracing Stance 2, giving her a total of 37 Defense and 41 Resistance while next to an ally and upon enemy initiation, effectively giving her mixed bulk of 77 and 81! This is just about enough for Sophia to ignore Great Flame's effect on a +DEF Myrrh (disregarding buffs). Quick Riposte guarantees follow-ups when an enemy initiates combat due to Sophia's poor Speed, allowing her to remove or severely chunk tankier enemies like Winter Tharja and Hoshidan Summer Micaiah, who would otherwise survive Sophia's counterattack with more bulk than she can deal with on the next turn. Res Ploy 3 lets Sophia use her massive Resistance to support herself and her team, allowing her to possibly break through tankier blue units with some support. Sophia's Special is Iceberg in order to take advantage of her high Resistance stat and her low Speed, since if her foe initiates combat and Sophia can counter with Quick Riposte active, Sophia's second counterattack will have an activation of Iceberg to back it up.

While this Sophia (and Sophia overall) is meant for Enemy Phase, it does fairly well on player phase as well.

This build appreciates great team support and a good Support Ally for Sophia to grant her the best overall performance. Corrin (M) is an excellent choice, as he can grant at least +6 to ATK/SPD/DEF/RES while next to Sophia (if he has a Yato refined for the effect and an S-Support with her). Mikoto and other healers do wonders for Sophia's bulk and also allow her to keep Quick Riposte up for much longer than normal. Mikoto works especially well since a set using Wings of Mercy and Infantry Rush synergizes with Sophia's double Bond skills and massive Attack stat while they are active. Sophia's normal good allies also work extremely well with this set.

Another version of this set favors using double Atk/Def Bond 3 instead of one Atk/Def and one Atk/Res. In this set, the majority of the set remains the same, except the A-Slot skill is replaced with a second Atk/Def Bond 3, the optimal boon/bane is +DEF -SPD, and Sophia's special is changed to Bonfire. Said alternative set grants +10 Attack/Defense when Sophia is next to an ally, making her an obscene physical tank. Luna is also another powerful option for either the magical tank or physical tank set that lets Sophia break through walls with high magical bulk. Like Iceberg or Bonfire, it will be activated on Sophia's second counterattack if she is doubled with Quick Riposte active, but is much less reliable against enemies with low Resistance. Heavy Blade can be used as Sophia's Seal or A-Slot skill to let her charge her special faster, which grants her the opportunity to use skills like Astra, Ignis and Glacies more effectively. Note that although Sophia can use Aether, she cannot activate it in the first round of combat without assistance from someone with Infantry Pulse or usage of Quickened Pulse, although at best she will have it ready for the next turn's Player Phase.

This set turns Sophia into one heck of a tank for both melee and ranged units, owing to her Eternal Tome refine which grants her Bracing Stance 2 (+4 Def and Res on foe initiation).

With Sophia's decently high Def, Close Counter is a perfect A skill for her, allowing her to counter most Green units (especially armored ones) HARD and also allows her to tank Colorless bow units really well with the Eternal Tome having a Raven tome effect.

Quick Riposte is a must for Sophia's B skill given how pathetically low her speed is as it ensures she can follow-up her foes when they attack her.

The C skill and Seals are what makes this set interesting. With 29 Res, Sophia can make good use of ploys. Thus, Atk ploy is the best choice for her as it complements her ability to be a tank. Atk smoke is chosen as it further cripples the opponent as they'll struggle to do any damage to your team (save for a certain few.

The best boon-bane set for Sophia is +Atk/-Spd as since Sophia speed is way too low to salvage, a speed bane won't really hurt her at all. With an Attack boon, Sophia can reach 50 attack with her weapon which is fairly high enough to deal serious damage unto her foes.