Valter

Low Health, High Mobility
Since Valter is gonna lose a ton of health with his weapon, you can take advantage of his low health situation, using health reliant skills to make him even more powerful. The main issue is that his low health makes this play risky, but also really engaging.

This set in particular takes 100% advantage of mobility and allows Valter and his allies to soar around the map without care, being as aggressive and evasive as you want.
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Alternative skills:
Pivot - If you want him to move around instead of your allies.

Moonbow - If you just want to abuse the special trigger lowering so that his second attack will always be Moonbow triggered.

Iote's Shield - If you don't want about archers neutering his mobility even more so than they do, this will cushion the blow.

Fury - If you are insane enough to use this in conjunction with his Lance, you will most definitely lose all your health quick, but this is only recommended if you want to abuse low health builds such as Brash Assault 3

Defiant Skills (Mostly Attack) - He's gonna easily get under 50% HP, you mid as well take advantage of it!
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Supporting Skills:
This one is all dependent on your team in question, so this is completely up to you!
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Counters:
Ranged units - Units like Nino, Reinhardt, and any close archer (Especially Takumi with Close Counter + his natural bulk) will give him a tough time, and limit his mobility, clipping his wings in a sense.
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Strengths:
Highly mobile for Valter and team mates
Quite fun
Allows his low health to be a benefit than a hindrance

Weaknesses:
Ranged units
Relies on being on low health to let this gimmick to work
There's probably way more optimal sets

Cursed Lance would be his preferred weapon give him half a Fury 2, with a +4 to his Atk/Spd while taking 4 damage after combat, and a Killer/Slaying Axe give his special trigger -1. Another option would be a Brave Lance+ with a good A Passive and or Hone Flier it can cancel the -5 Spd on Brave Lance.

Reposition is a definite on him to help other units reach spaces faster by going over terrains they can't cross.

For his Special Luna and Galeforce would be great due to his -1 on cooldown for his Curse Lance. Sol or Noontime could be another option if you want to run Fury 3 to help him survive due to him taking 10 damage after combat. With his good Def Bonfire or Ignis could be run instead give him either a 17 plus or 27 damage.

With Swift Sparrow 2 it will give Valter a +6 to both his Atk and Spd combined with his Cursed Lance. Another option for his A Passive would be either Darting Blow 3, Life and Death, Heavy Blade 3, or Death Blow 3.
Darting Blow 3 would help his speed will boost up to 37 combined with a Hone Fliers and Cursed Lance effect his speed gets up to 45 in battle allowing him to not get doubled and to double most of the rosters. Life and Death would be another great passive if you want to sacrifice his Def to 29 but give him more Spd and Atk. With his Atk of 48, while having his Cursed Lance equipped, Heavy Blade would help out in reducing his special's could down. Death Blow 3 will be equally as Darting Blow 3 with a Hone Fliers his Atk will be boosted to 60.

For his B Passive Lancebreaker3 would be good to deal with other Lance units. Other alternative would be Renewal 3, Draw Back or Hit-and-Run.
Renewal would be helpful for recovering out of battle with the 4 Damage after combat. Draw Back or Hit-and-Run would be great if he unable to kill and units or escape cross terrain infantry units cannot go over.

For his C Passive Panic Ploy 3 would be helpful and deal with most Horse Emblem teams. On a Fliers team having either Hone/Fortify Fliers could be run. And to help his Atk or Spd either Threaten Atk or Spd could help him during the battle.

For a Sacred Seal Quicken Pulse will reduce his special cooldown to a -2 at the start of the round with Cursed Lance. Speed +1 or Attack + 1 can help out with this Atk or Spd.

I choose to create this build based off of his well rounded base stat total with 31 spd (excluding his weapon) I believe Valter is a excellent candidate for heavy blade because it partners beautifully with his cursed lance reducing your cooldown by one already as well as giving you +2 atk/spd. Then this is where galeforce comes in, with quickened pulse heavy blade and his weapon it is easily accomplished in the first engagement. Also another good reason to go with this is because I feel fury is a bad pick when you'll be losing 10hp on top of combat damage. I wouldn't reccomend using this build if you are using Valter outside of a flier emblem team, because heavy blade only works as well being able to double consistently with Hone and goad fliers. This is also a very expensive build so I would recommend levelling Valter up from 3 star. I went back and forth on having the slot b skill include hit and run instead because it would go well with galeforce if you need to retreat after killing a unit on a Grand hero battle, arena etc. You are also welcome to swap out fortify fliers for Panic ploy if wish to save some sp and keep with his default c slot. I hope this build has inspired you to think up unorthodox builds like this one I personally will be building my Valter this way.

*Speed Seal????
This setup makes Valter a self sustaining unit.

The Cursed Lance gives Valter +2 to attack and speed and gives him a faster special trigger but in exchange, he takes 4 damage from any sort of combat.

A Speed Seal, the Cursed Lance and Darting Blow combined give Valter 40 speed and 50 attack.

Noontime is used because, along with the Cursed Lance, it has a cooldown of 2 for only 20% less health than Sol.

Panic Ploy is just filler, his C skill depends on the team he is being used on.

Warning:
You probably shouldn't give him Fury if you want to use the Cursed Lance, he would be losing 10 hp each round of combat and in most cases that is simply too much health to lose at one time.