Flavia

Weapon: Lioness Blade

Assist: Positional Assist / flexible

Special: Luna / Blue Flame / flexible

IVs: +Atk or +Spd / -Res

Passive A: Atk/Spd Unity / Atk/Spd Push 4 / flexible

Passive B: Spurn / Null C-Disrupt / Close Call/Repel / flexible

Passive C: Red Feud / Joint Drive Atk / flexible

Sacred Seal:  Atk/Spd Bond / Atk/Spd Solo / Kestrel Stance / flexible

• Flavia’s weapon, Lioness Blade, has several great effects, including the ability to ignore damage reduction from all skills other than Specials when she activates her own Special. This allows her to deal heavy damage on the counterattack and KO many annoying units like Brave Eirika and Legendary Sigurd, though she needs to watch out for Eirika’s pre-charged Moonbow build.  
• Lioness Blade accelerates Flavia’s Special charge by 1 on all attacks, hers and her foes, while her HP is above 25% and her foe’s HP is above 75%. As such, the ideal Specials to use are 3-cooldown, which is reduced to 2 by Lioness Blade, so that she can immediately counter with a Special after getting attacked. Luna is a great choice that adds damage proportional to 50% of her foe’s Def, while Blue Flame adds a guaranteed 25 damage so long as she is adjacent to an ally. Other options like Draconic Aura can also work. 
• The boon is flexible, with +Atk and +Spd both being good options for increasing damage and getting more reliable damage reduction, respectively. -Res is generally the best bane, as most threatening melee units hit Def. If she regularly needs to fight dragons, -Def could be preferable. 
• While Flavia has good Def, a B skill like Spurn allows her to take reduced damage from enemies she outspeeds, up to a 40% reduction if she is 10 points faster. This helps her survive much more reliably, in addition to providing a slight damage boost of +5 if she activates a Special while below 75% HP. Other damage reduction skills like Close Call and Repel will also work, but are generally worse than Spurn. Null C-Disrupt can also be used to allow Flavia to counterattack foes with effects like Firesweep Lance+ or when she is under the effects of Flash+, but she is more vulnerable to being KO'd by high damage single hits. 
• The C skill is flexible. Flavia’s base Red Feud is an interesting option that disables all skills of all units other than her foe if she’s in combat against a red unit, such as Drive Attack or Caduceus Staff, and disables support skills from red units if she’s in combat with any color of unit. She can also swap it out for something like Joint Drive Atk or Joint Drive Spd for a more consistent stat boost. 
• The seal is flexible. Atk/Spd Bond provides a boost of 5 Atk and Spd if she is adjacent to an ally, so it pairs well with Blue Flame. Atk/Spd Solo boosts her Atk and Spd by 6 if she is not adjacent to an ally, so it can be a better choice on teams where adjacency is too risky. Kestrel Stance provides a slightly lower stat boost to Atk and Spd of 4, but without any restrictions on positioning. 

Weapon: Lioness Blade

Assist: Positional Assist / flexible

Special: Luna / Sol / flexible

IVs: +Atk or +Spd / -Res

Passive A: Distant Stance / Distant Counter

Passive B: Spurn / Close Call/Repel / flexible

Passive C: Red Feud / Joint Drive Atk / flexible

Sacred Seal:  Mystic Boost / Atk/Spd Solo / Kestrel Stance / flexible

• This build is similar to the previous one, except with less of a focus on specifically melee enemies, and instead uses an A skill like Distant Stance or Distant Counter to counterattack at all ranges. 
• The choice of Special depends somewhat on the use case. If Flavia will only be fighting one or two enemies inside one turn, then a damaging Special like Luna is a good choice to potentially allow her to KO even bulky foes on the first counterattack. If she needs to survive several combats, then it may be better to use a healing Special like Sol, but she will struggle against high Def opponents, especially Blue units. 
• Mystic Boost is a good choice of seal, as it heals 6 HP after combat, as well as disabling Wrathful Staff and similar effects and any effect that “targets the lower of Def or Res”. Atk and Spd-boosting seals like Atk/Spd Solo are still good choices as well, depending on whether Flavia needs better healing over multiple combats, or more power in specific encounters.