Ryoma

Me: get pity broken by Lobster King himself, oooh yeah
Also me: check gamepress, Lobster King has been out-metaed by Ayra and Zelgius (sad face) Puts Ryoma on bench for few months
FEH: Releases Brazen Atk/Res seal, oooh yeah
Me again: IT'S TIME FOR DOUBLE BRAZEN

So, this Ryoma build can be used regardless of boon/bane, although a +Atk is nice (mine is +Spd). With Brazen Atk/Def in A slot and Brazen Atk/Res in S slot, Ryoma basically walks up to anyone and hits them once, gets hit back, and enters DOUBLE BRAZEN VANTAGE mode, which gives him whopping +14 Atk/+7 Def/+7 Res.

Any unit with low Def will get eaten before they actually pull off an attack (especially if Glimmer is charged up), and even if they do, Ryoma is incredibly tanky due to the added Def/Res and should survive a few hits.

Assist and C slot are flexible, although I found Savage Blow in C slot does help soften enemies up enough to allow for easier Vantage OHKOs.

Serves primarily as a ranged counter for every color except blue, secondary role is buffing s-class support partner F!Corrin with a +5 in speed so she can easily destroy any blues in his way, minor role, red and green replacement counter should Lucina ever go down.

Use summoner support to get above stats. In this build, you initiate an enemy, ORKO (hopefully), and then proc heavy blade, which will proc galeforce if a melee unit. Then move into range of green or red mage and vantage will OHKO. If Ryoma has enough hp, it can take a hit from the mage. Is +1 merged

If you Summoner Support Ryoma and equip an Attack +3 Sacred Seal his stats than become 42/55/40/29/23. This build is meant for Ryoma to use his good speed and attack on the attack phase when able to be in Desperation range. Swift Sparrow will put his attack and speed to 57/42 or 59/44 if Summoner Supported. This build will almost determine a one round knock out if used correctly. Lord have mercy if ever a Vantage Seals gets released.

S Support:
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HP: 46
Atk: 55
Spd: 43
Def: 32
Res: 23

S Support and +10 Merge
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HP: 50
Atk: 59
Spd: 47
Def: 36
Res: 27

Although Ike holds his niche of being a better Steady Breath user, Ryoma far surpasses Ike as a Fury user. By abusing his high speed for follow-ups and the extra charge from Heavy Blade, Ryoma charges his Luna in one battle and with Wrath ORKOs almost anything he can double. Alternatively, you can also use Aether if you want to allow Ryoma to self-heal (or for arena points), which increases his overall longevity but makes him a slightly worse duelist.

Fury is Ryoma's best A-skill for this build. All other A-skills force him to lose either bulk or power, which Ryoma needs to function on both player and enemy-phase.

Wrath lets Ryoma gain much more damage off his procs, making his Luna insanely powerful and Aether's recovery more prominent. Vantage can be used as a budget as if he cannot double his opponent, he can proc his Luna on his Vantage turn.

C skills are always variables. As a stand-alone unit, Spd Smoke allows Ryoma to double more units and makes him a better duelist all around. Atk Smoke can also be considered if you're not worried about Ryoma not doubling his opponents. Other options include Hone's, Threaten's, Panic Ploy (if S Supported and merged) if the Smoke's are too expensive.