Yen'fay

Weapon: Carnage Amatsu

Assist: Any high-scoring Rally+ / Harsh Command+

Special: Ruptured Sky / Aether / Vital Astra / Blue Flame / Galeforce

Passive A: Atk/Spd Solo 4 / Atk/Spd Unity / Kestrel Stance 3 / Atk/Spd Ideal 4 / Atk/Spd Push 4 / High-Scoring Spd/Def or Spd/Res Equivalents

Passive B: Null Follow-Up / Lull Spd/Def / Lull Atk/Spd 

Passive C: Joint Drive Spd / Atk/Spd Menace / Flexible

Sacred Seal: Steady or Darting Breath / Atk/Spd Solo / Spd/Res Solo

• Because Yen'fay has access to a PRF weapon in Carnage Amatsu (meaning his weapon slot gets a high SP value) and can reach a respectable scoring bin of 185+ with his high BST, he can be a great choice as a scoring anchor and core unit in modes that utilize Arena scoring. The goal of this build, then, is to optimize scoring using high SP skills in every slot possible while still maintaining some competitive performance.
• Carnage Amatsu, again, is an obvious must for scoring and combat performance. With it, Yen'fay gets access to damage reduction and the ability to counterattack ranged enemies, which conveniently opens up Yen'fay's A and B slots to a wide variety of options — as long as they score 300 SP+. Many of them are even available in Normal Compile Manuals as well.
  • Ideally, you should run any high-quality Atk/Spd boosting A slot skill you can offer (besides Bonds) in order to maximize damage and follow-up potential while contributing to the Speed-based reduction from Carnage Amatsu. Atk/Spd Solo 4 notable above all other choices as it synergizes with the solo condition required to trigger the damage reduction effect. It also grants Yen'fay a degree of phase flexibility so he isn't relegated to simply waiting for attacks, and a lot of units that can be used to inherit Atk/Spd Solo 4 also happen to come with Null Follow-Up, which is essentially Yen'fay's best B-slot option. 
  • If Null Follow-Up isn't in your cards, you can fall back on the handy Lull skill line to maintain scoring goals while granting a stat advantage in Yen'fay's favor. Lull Spd/Def or Lull Atk/Spd are preferred above all else to discourage the enemy from beating the damage reduction Speed check.
• A high-scoring Rally or Harsh Command+ is pretty much standard on Arena builds, and Yen'fay isn't particularly picky with which ones he has. Make sure to avoid redundant Rally boosts when picking other assist skills on Yen'fay's fellow Arena core teammates. 
• Yen'fay also isn't picky with the Special he has as long as it has a 750 SP cost. Performance-wise, it would be ideal to have Ruptured Sky, Vital Astra, or Blue Flame due to being quicker to activate than Aether or Galeforce, both of which are great budget alternatives from the common pool anyway.
• If you managed to provide the maximum possible SP skill to all slots possible at this point, you can relax SP requirements on the C slot as long as it's 360 SP or over — running anything more than that won't have any effect on Yen'fay's score bin. That being said, Joint Drive Spd or a Speed-based Menace are fantastic endgame C slot options. Due to their SP value, they also maximize the C slot's scoring contributions if you can't max out any of the previous slots for any reason. 
• Finally, top off the build with a good Sacred Seal: this will be either a Breath skill to quicken the activation rate of Aether, Blue Flame, or fire off Ruptured Sky instantly on every enemy initiation, or a Solo skill (Atk/Spd or Spd/Res preferred) to further strengthen Yen'fay and encourage his weapon's damage reduction conditions. 

Weapon: Carnage Amatsu

Assist: Positional Assist

Special: Noontime / Moonbow

Passive A: Atk/Spd Solo 4 / Kestrel Stance 3 / Atk/Spd Ideal 4 / Atk/Spd Push 4 / Atk/Spd Unity 

Passive B: Null Follow-Up / Lull Spd/Def / Lull Atk/Spd 

Passive C: Time's Pulse / Spd Smoke / Atk Smoke / Various Rouse or Menace skills

Sacred Seal:  Atk/Spd Solo / Steady or Darting Breath / Spd/Res Solo

• This is virtually identical to the Arena build, but since we're prioritizing combat and general gameplay over maximizing scoring, Yen'fay's skill choices are considerably more relaxed and efficient here. The game plan is similar and simple as ever though: build up Yen'fay's Speed and Attack to increase his offensive potency as well as feed Carnage Amatsu's damage reduction effect. 
• A major change here is the choice of Special and how Yen'fay is going to activate it. Due to the lack of Slaying or Special acceleration effects present, this build plays it safe with 2 CD Specials — Noontime for impromptu healing and longevity, or Moonbow for short and sweet burst damage. 
• Though not entirely necessary for the build to succeed, Yen'fay will prefer any form of accelerated charge in order to instantly fire his chosen Special off on the enemy's attack. You can achieve this by:
  • Running the Steady Breath Sacred Seal on Yen'fay himself at the cost of other good Seals
  • Running Time's Pulse to effectively shorten these Specials to just 1 CD at the start of every turn; note that if Yen'fay is being attacked multiple times in one round, he will only be able to trigger his Special every other attack after his first.
  • Finally and more efficiently, teaming Yen'fay up with an ally that can provide acceleration support, such as Brave Lucina or Thórr or any unit with Infantry Breath. 
• Besides Time's Pulse, Yen'fay can utilize cheaply-acquired Smoke skills to improve his matchups. Spd Smoke improves the potency of his Dodge effect while opening more follow-up attacks, while Atk Smoke is a general catch-all filter for incoming hits. Additionally, you can run a variety of Rouse-type skills to further encourage Yan'fey to keep up Solo conditions while being rewarded with stat buffs to himself.
• Again, passive skill elements from other builds remain largely the same for attacking optimization.
  • You really can't go wrong with double Solo skills since it provides a great number of stats in both phases and Carnage Amatsu requires Yen'fay to be away from allies anyway. Lacking that, there are many great Atk/Spd alternatives with different conditions out there as long as you avoid the Sparrow or Bond-type lines. 
  • In the B slot, Null Follow-Up evens the playing field by disabling otherwise gamebreaking follow-up manipulation skills. If you want to prioritize raw stat matchups, Lull Spd/Def or Lull Atk/Spd helps maximize the Speed-based damage reduction from Yen'fay's PRF while improving damage or bulk respectively. 

Weapon: Carnage Amatsu

Assist: Positional Assist

Special: Moonbow / Noontime

Passive A: Atk/Spd Unity / Kestrel Stance 3 / Atk/Spd Solo

Passive B: Null C-Disrupt

Passive C: Joint Drive Atk / Pulse Smoke / Atk Smoke

Sacred Seal:  Atk/Spd Solo / Mystic Boost

• Yen'fay's builds are becoming a bit samey as you can see, but the primary differentiation between this and other builds is Null C-Disrupt. Utilizing it allows Yen'fay to counterattack enemies running cancellation attacks such as Dazzling Staff or Windsweep, both of which are common primary tools used on defending units in Aether Raids.
• Therefore, this build is great for specific use on Aether Raids Offense teams. That being said, for other modes or Aether Raids Defenses that DON'T use counterattack cancellation techniques, it would be best to run other builds (preferably with Null Follow-Up).
  • In such cases, Yen'fay can simply outspeed Windsweep enemies so that Windsweep no longer triggers, plus other modes typically give you more wiggle room to run specific Dazzling healer counters rather than have Yen'fay deal with them. 
• As an omnitank, it's imperative that Yan'fey tanks and ORKOes any matchup he'd faced against to prevent Lift scoring drops. Therefore, a little more emphasis on Attack is placed here compared to other builds.
  • Joint Drive Atk, for instance, is a preferred self-boosting C slot choice that will likely come into play as Yan'fey positions himself for ally support skills that also work "within two spaces".
  • On that previous note, Legendary Byleth, in particular, is a good recommendation as a support ally as his Professorial Text grants the missing Null Follow-Up skill to Yen'fay, reducing possible incidents of losing to follow-up manipulation skills like Valentines Líf's Þjálfi.  
  • Moonbow is a preferred Special for short and predictable burst damage, and should be paired with Mystic Boost for some sort of sustainability. Atk/Spd Solo + Noontime is also an option if you prefer stronger non-Special hits and possibly higher heals. 
• With a focus on omnitanking, more specialized A-slot skills are considered. Atk/Spd Solo will still do just fine for mixed-phase and modes outside of Aether Raids, of course. However, the penalty reversion effect of Atk/Spd Unity or the enemy Special delay from Kestrel Stance 3 will definitely be preferable in most cases.  
• Though Infantry Pulse teams are lowering in popularity, Pulse Smoke is still a solid option to stall incoming Special hits and check these types of teams whenever you do encounter them. On a budget, Atk Smoke will do just fine in softening incoming attacks.

Weapon: Carnage Amatsu

Assist: Positional Assist

Special: Noontime / Moonbow

Passive A: Kestrel Stance 3

Passive B: Close Call 

Passive C: Spd Smoke / Atk Smoke

Sacred Seal:  Atk/Spd Solo / Spd/Res Solo / Steady Breath

• We actually discourage running this build for a few reasons.
  • Having Speed-based damage reduction applied in two slots (that is, Carnage Amatsu and Close Call here) builds up the total amount of reduction multiplicatively. Conclusively, Yen'fay's maximum damage reduction will only amount to 64%, which, while pretty good and certainly an improvement from the base 40%, is considered a major diminishing return. 
  • By running Close Call in the B slot, Yen'fay improves slightly on his damage reduction effects while incurring an opportunity cost: adding extra effects that directly addresses his checks and counters. Passing up other effects, especially Null Follow-Up, overall makes him a less versatile, less improved, and more countermeasure-vulnerable (please kill us faster, y!Innes) fighter. 
  • Kestrel Stance 3 is actually a pretty neat, near-optimal skill to have on Yen'fay. It's a natural progression from his default Kestrel Stance 2, and while it largely limits him to enemy phase only, the sizeable Atk/Spd swing and Guard effect grants a layer of protection versus enemies attempting to break through with a Special hit. Its only disadvantage — and this is a pretty big one in situations where he's forced to initiate combat —  that it applies only in Enemy phase, making Yen'fay pretty phase-inflexible.
  • Saccing his sister — the only unit in the game who has Kestrel Stance 3 at this time of writing for a tragic twist of fate — is kind of screwed up, man.
    • ...but if you MUST, might as well grab her also-available Close Call 3 which is a fairly rare and somewhat useful skill at the end of the day.