Ninja Corrin (M)

Weapon: Ninja Naginata+

Assist: Positional Assist

Special: Luna / Dragon Fang

Passive A: Swift Sparrow / Flashing Blade 4 / Atk/Spd Solo / Life and Death / Atk/Spd Ideal / Death Blow / Atk/Spd Push 4

Passive B: Special Spiral

Passive C: Atk/Def Menace / Def Smoke / Spd Smoke / Joint Drive Atk / Atk/Spd Menace

Sacred Seal: Blade Session / Flashing Blade / Swift Sparrow / Life and Death / Death Blow

• This builds off Corrin's fairly good base kit and playstyle. We start with the backbone defaults:
  • Similar to conventional Brave Lances, Ninja Naginata is a hard-hitting tool which deals double hits in one strike. In exchange for lower MT per hit, Corrin gains great blitz damage and a method of charging his Special more quickly (as a result of consecutive attacks). As a nice plus, Ninja Naginata also grants up to 4 true damage based on how far Corrin's Speed outmatches his enemy's.
  • Special Spiral's inclusion on Corrin's base kit is both a blessed wonder and a great method of speeding up his Specials. Following the activation of his first chosen Special, Special Spiral immediately charges its cooldown by -2, allowing it to activate even more quickly after Ninja Naginata's double hits. Essentially, use Naginata to quickly charge up Corrin's Special, with Special Spiral making it even easier after every activation as long as the match allows. 
• Because of the quick Special charging offered by Special Spiral and Ninja Naginata, Corrin can consider a few fancy Special options. 
  • Luna is easiest to recommend; assuming no Guard effects are in place, it's guaranteed to fire off after Special Spiral on Ninja Naginata's second hit. The fact that it cuts through enemy Defense also largely makes up for Ninja Naginata's low MT per hit. 
  • Dragon Fang is generally a more powerful option, but requires the accelerating charging of Flashing Blade to fire off in a single round. Note that against faster enemies, this may fail to activate in a timely manner as Flashing Blade won't be in effect. 
• The rest of the build comes down to strengthening Corrin's offenses.
  • The most direct method is running any initiation-friendly Atk/Spd boosters you can offer in the A-slot and Sacred Seal (barring Flashing Blade's possible inclusion). You can opt, for example, to simply improve Corrin's base Swift Sparrow into its third variant, or go all-in on the all-or-none nature of the build by running Life and Death.
  • Atk/Def Menace is one of the best ways to swing matchups favorably towards Corrin's side by buffing his own attack versus lowering the enemy's Defense. Similarly, Atk/Spd Menace increases the likelihood of Ninja Naginata achieving its maximum true damage as well as a quadruple attack. Should those not be available, other C-slot enhancements such like Joint Drive Atk or the budget-friendly Def or Spd Smoke are also on the table.

Weapon: It's Curtains…+ (+Spd) / Ninja Naginata+ / Ninja Yari+

Assist: Positional Assist

Special: Galeforce 

Passive A: Flashing Blade / Swift Sparrow / Life and Death / Blade Session / Atk/Spd Push 4

Passive B: Lull Spd/Def / Wings of Mercy

Passive C: Time's Pulse / Even or Odd Tempest / Savage Blow / Smoke skills

Sacred Seal: Blade Session / Flashing Blade / Quickened Pulse / Swift Sparrow / Life and Death

• With a multitude of lances and infantry tricks up his ninja smallclothes, Corrin can whip up an effective offense setup based on Galeforce, enabling him to chain multiple actions in a turn. The skill combinations shown also suffice for damaging Specials, but for neatness's sake we'll cover what exactly makes Galeforce tick nicely on Corrin.
• Corrin's access to lances yields a few neat specialty choices:
  • It's Curtains… is still a standard on most one-hit Galeforce units in Aether Raids Offense as it immediately reduces the cooldown of Galeforce down to 3. From there, activating it becomes a lot more flexible, especially with the help of Pulse skills. 
  • For a stellar boost in Speed (and thus a better chance at quad-attacking and activating Flashing Blade) while retaining the consecutive hitting mechanics of Ninja Naginata, Corrin can opt to steal his fellow Ninja Navarre's Ninja Yari. Seeing how expensive this inheritance choice is and easy it is to provide outsourced Speed boost support, though, you may opt simply to keep Naginata, which deals better damage against units Corrin outspeeds.
• Flashing Blade is a way to gain more Special charges upon hitting, and given Galeforce's very high cooldown of 5, it'll more than likely find itself in either the A-slot or Sacred Seal. In any case, Corrin should have additional Atk/Spd amplifiers to strengthen himself; Blade Session, in particular, is typically a great choice as Corrin typically won't move before other allies on his team and achieves its maximum conditions (+9 Atk/Spd). 
• Time's Pulse is universally good on every variation of this build, allowing Corrin to self-sufficiently charge his Galeforce by -1 without relying too much on other Pulse sources. If you can precharge Corrin through other ways (such as Ostia's Pulse II or Infantry Pulse), though, Even or Odd Tempest are stellar C-slot choices that increase his movement and initiation options. Smoke-style skills are perfectly viable and budgetable substitutions if these skills are out of reach or unnecessary.
• For general gameplay purposes, Lull Spd/Def would be best as it actively improves Corrin's matchups in terms of Speed and damage. For Aether Raids Offense, Wings of Mercy takes center stage since it allows Corrin to teleport to other damaged allies. This is a particularly important utility for Galeforce teams so as to overcome Corrin's average movement options and continue the chain of actions in one turn.
• Some effective skill combinations are as follows:
  • It's Curtains + Flashing Blade + Wings of Mercy + Time's Pulse (One-hit Galeforce)
  • It's Curtains + Wings of Mercy + Time's Pulse + Quickened Pulse (One-hit Galeforce with no Flashing Blade activation required)
  • Ninja Naginata or Ninja Yari + Flashing Blade + Time's Pulse or Quickened Pulse (Galeforce activates as long as two hits land and Flashing Blade is active)

Weapon:Springy Lance+ (+Spd) / Slaying Lance+ (+Spd) / Spirited Spear (+Spd) / Steadfast Lance (+Spd) / Flowing Lance+ (+Spd)

Assist: Positional Assist

Special: Noontime / Moonbow / Sol

Passive A: Atk/Spd Solo / Distant Pressure / Distant Counter / Atk/Spd Unity / Atk/Spd Ideal / Atk/Spd Push 4

Passive B: Spurn / Frenzy / Close Call / Repel

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

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

• If you don't mind that Corrin will do the speed tanking job with a slight stat and PRF disadvantage to other competition, this role is completely within his talents. Notably, what he DOES have over units like Legendary Dimitri and Nótt is the fact that he's more easily merged, isn't locked to single seasons (as he can be blessed to either Light or Astra for AR Offense tanking), comes pretty close to their statlines regardless, and running refinable weapons actually grants him higher HP, which scales well with the damage reduction of his chosen B-slot. 
• After seeing so many infantry units replicating this strategy, the build pretty much writes itself out: Raise Corrin's Speed as much as possible to maximize Spurn, Frenzy, Close Call, or Repel's Speed-based damage reduction (up to 40%), which meanwhile increases his follow-up potential and simply makes him a great dual-phase unit all-around.
  • You can pretty much choose from any of these four skills, as their secondary effects don't really matter compared to the damage reduction. Spurn is slightly preferred, though, as it can potentially squeeze in some extra damage at lower HP and synergizes nicely with recoil-induing A-slots. Likewise, Frenzy's Desperation effect may prove sitationally clutch, especially where Corrin has to play a bit more aggressively or risk getting finished from a counterattack. 
• Springy Lance is pretty much the number one weapon for this build as it's one of the few lances that actively stacks Attack and Speed; its condition -- engaging enemies with over 75% HP -- is also pretty much guarunteed all the time while giving him penalty neutralization in those stats (meaning he can't get bogged down by sudden Attack or Speed penalties). Since it's locked to a seasonal demote, however, Corrin can run some other accessible options:
  • Slaying Lance is one of the most common weapons to inherit and refine, and its CD -1 effect allows Corrin's chosen Special to activate with increased frequency. It's preferred when utilizing Sol since it can also be paired with Steady Breath to get Sol ready on every enemy counterattack. Though, if you're sticking to a 2 CD Special like Noontime, it's usually better to run Spirited Spear as it also accelerates special charges on enemy attack while providing a nice stat boost to Atk and Def.
  • Like Spirited Spear, Steadfast Lance and Flowing Lance can grant some nice Atk/Def swings, strengthening Corrin's general physical durability while requiring him to abide some easy positioning caveats. Flowing Lance is also seasonally locked until its generic version comes along, keep in mind. 
• Corrin's A-slot mostly depends how and where you use him. If used as a solo carry, Distant Pressure or Distant Counter allows him to counterattack as many units as possible. Keeping him grounded as melee-only generally makes him more combat-competent, though, as he can benefit from the extra Atk/Spd from skills like Atk/Spd Solo, Push, Unity, and so on. Ideally, you can run a Far Save armor ally to handle all ranged combat while Corrin handles melee. 
• For the most part, a Spd-based Solo skill in the Sacred Seal will improve on Corrin's offensive competency by contributing to his B-skill's damage reduction and improving his Speed matchups. Steady Breath was mentioned before as a good way to immediately ready Specials on counterattack, but is recommended with  additional Speed Spurs / Drives from other allies. 
• Corrin's C-slot can be filled in with anything that helps out his tanking. On a budget, Atk Smoke or Spd Smoke do well to debuff groups of enemies and prime Corrin better in mob matchups, while a Joint Drive of your choice is notable for working without requiring combat while also providing stat buffing for other allies. Atk/Spd Menace targets single enemies from four spaces, which can be advantageous but may hit the wrong thing at times. 

Weapon: Slaying Spear+ (+Spd) / Springy Lance+ (+Spd) / Candy Cane+ (+Spd) / Flowing Lance (+Spd)

Assist: Any High-SP Rally / Harsh Command+

Special: Ruptured Sky / Aether / Blue Flame / Galeforce

Passive A: Atk/Spd Solo 4 / Distant Counter / Swift Sparrow 3 / Atk/Spd Push 4 / Atk/Spd Ideal

Passive B: Null Follow-Up / Spurn / Lull Atk/Spd / Special Spiral / Other Dodge skills

Passive C: Joint Drive Atk / Atk/Spd Menace / Atk/Def Menace / Joint Drive Spd / Other 300 SP C skills

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

• With a high base stat total which puts Corrin in the very high 185 BST scoring bin, Corrin can serve as a scoring anchor in your Arena teams or other settings that utilize Arena scoring. Fortunately, he's also a competent unit by himself but will require maximum SP skills to achieve his max scoring potential.
• As far as weapons are concerned: 
  • The prevalence of Fallen Edelgard and other overbearing armored enemies makes Slaying Spear a conventionally good option. It reaches its full potential with Null Follow-Up, allowing Corrin to bypass otherwise annoyingly restrictive enemy skills such as Twin-Crest Power and Bold Fighter. This also makes Corrin a reliable pick for Arena Assault as a consistent armor check, no matter his merge level.
  • For general performance, you may instead be interested in a nice list of stat-beneficial weapons. Springy Lance, for instance, improves general offense by granting a hefty Atk/Spd swing of +5 as well as penalty neutralization to those stats. Candy Cane and Flowing Lance both invest in Atk/Def instead while also granting enemy Special delay or an essential Lull effect, respectively.
• There's, fortunately, a continually growing list of 300 SP A-slot skills besides Distant Counter (or any of its improved variants if you're able to afford it), which Corrin uses well enough to counterattack the majority of enemies but at the cost of additional stats. So as long as you inherit their maximum 300 SP variants, Atk/Spd skills such as Solo, Push, Ideal, or Swift Sparrow will do just fine in stacking up Corrin's scoring and offensive presence. 
• Without a unique weapon, Corrin must also take up a 300 SP skill in his C-slot to maximize his scoring. Most will do fine, though Joint Drive and Menace skills involving Atk, Def, or Spd will benefit Corrin himself while also providing varying levels of support for other allies. 
• As is basically the optimal path on any Arena core unit, Corrin will have to take up a high-level Assist and Special skill.
  • This pretty much flows down to any Rally+, Rally Up+, or Harsh Command+ you can offer in the Assist. Rally Def/Res+ is notably available for near-immediate redeem on Normal 2's New Year Anna in Compile Manuals. 
  • Corrin may be slightly pickier for his Special as Aether and Galeforce are free-to-play accessible but a bit impractical due to their high cooldown, and the opposite applies for Ruptured Sky and Blue Flame and their lower cooldown but being generally rare to come by. 
• Null Follow-Up has been discussed before as a good way to tame enemies with follow-up blockage or guaranteed follow-up attacks, and will generally do well on any build. Lacking that or wanting to prioritize a little more survivability, Corrin can instead consider Lull Atk/Spd or anything from the Dodge skill line (that is, Spurn, Close Call, and the other variations). His default Special Spiral also helps get his Specials ready to fire after their first activation, though note that it's a completely dead slot when used with Galeforce (that activates out of combat). 
• When all's been said and done, top off the build with a 240 SP Sacred Seal. A Solo skill of choice will generally do fantastic by providing a great immediate stat boost with practically no setup, while Steady Breath helps get those impractical 5 CD Specials ready with more reasonable timing.