Seliph

IVs: +HP/Res, -Spd

While Sigurd shrugs off magical attacks, Seliph his son has been left in the shadows and often been victim to them. With the weapon refinery, Seliph finally has access to the magical Divine Tyrfing, and this build will expand on its capabilities.

One of the reasons Seliph struggles with surviving even with Divine Tyrfing is because of the lack of Crusader's Ward. As such, Seliph needs as much bulk, real or artificial, as he can possibly get. +HP is taken for this purpose due to being able to use Panic Ploy to essentially create pseudo bulk against foes such as bladetome users by crippling them. His a and b slots are the standard fair Distant Counter with Quick Riposte, which are combined with Bonfire to maximize his counterattack damage.

If running this build, Seliph greatly appreciates either Deflect Magic or Distant Def 3 as seals, due to them increasing his still subpar survivability against magic.

I thought i would get ahead of the game with weapon upgrading being an upcoming mechanic, with the first set of Seliph using his father's divine tyrfing. Access to this weapon alone pushes seliph a few spots up on the tier list, but this set is the most optimal build i could think of. His high hp and def lead to him being able to bait melee units and easily survive the mages that once struck fear in his heart. Deflect magic is taken for synergy with his weapon to minimize incoming magic damage. When baiting, if he is doubled, he has 37 def and 47 hp and a guarunteed Aether to heal up the damage. The reason he has atk smoke is to be able to take a hit from a mage and bait a melee unit at the same time, applying atk-7 to whichever unit attacks second.

Specialized, mostly meme set Seliph build designed to "counter" Reinhardt via Divine Tyrfing, Deflect Magic, Shield Pulse, and Aegis.
Passive A is flexible; I have Distant Counter so he can be an answer to other mages or bait out powerful Bladetome users with squishy defense. Options such as Fury, Distant Defense, Steady Breath, or Warding Stance can work well here.
Passive B can be substituted for any number of things: Vantage can be taken for the classic Distant Counter + Vantage combination, Quick Riposte to patch up his speed weakness (it is his lowest stat, so it's advisable to dump it), or specialized options such as Renewal, or Swordbreaker.
Passive C infantry pulse is taken for my personal team consisting of three other infantry units, but is otherwise flexible. He's got 32 res so perhaps you could make use of a Ploy.
Seal taken is Deflect Magic for Reinhardt. Offensive builds may want to use Brash Assault, or for extra utility, the Atk Smoke seal.
No reason not to use Divine Tyrfing, Support skill is always flexible. Special taken here is Aegis, but an offensive build will want a skill like Draconic Aura or either Bonfire or Iceberg, depending on your choice of Boon/Bane. Now, assuming you have an infantry pulse unit on your team it can be up on turn 1 and allow you to duel Reinhardt (provided you are at equal merge levels; the calcs I ran were for a 4 stars +10 Seliph vs. a 5 stars +10 Reinhardt). Sacred Cowl can be taken for the same effect, minus the infantry pulse. Lastly, in order to truly duel Reinhardt, he'll need a res buff, and at least a C support, standing next to someone holding the Spur Res seal (even if it's level one it will work).
The reason Aegis is taken over Sacred Cowl is because: https://imgur.com/oFPiGAp

By completely scrapping his base kit, this Seliph is now able to deal out considerable damage during player phase and, when his HP hits that <40% threshold, he's able to quad. He has decent enough Def to make it work, but it's still a niche and expensive build.

This Seliph build may seem eccentric at first, but fret not, decking out your Seliph with these skills will help push him to the top of his potential. While Wo Dao+ seems like an odd choice for a non-speedy sword, it actually sees massive mileage with the kit I've given Seliph. With Swordbreaker, Seliph annihilates all sword units in the game (with the exception of armors), even the newly introduced problems called Sigurd and Ayra. With a +Attack nature, Seliph's attack even with a Wo Dao+ will hit a high 53 with Fury and no merges. This small attack dip does come at the cost of a sword with a way better effect than Tyrfing. (Geez Seliph, why couldn't you be like your father in that regard at least?).

With 53 Attack, Seliph with a Wo Dao+ and charged Moonbow can sincerely pack a huge punch, and even dent some tankier blues.

With 47 HP, the Panic Ploy 3 seal will hit everyone it needs to (low HP bladetome mages, Horse Emblem, a good portion of Flier Emblem.).

The C Skill and Support Skill are loose for the most part. Threaten Spd 3 is actually a little nifty to help out the rest of your team. If you willed Seliph to inherit SOMETHING from his father, you could put Speed Smoke 3 here.