Haar

Ah yes. Haar.
A very lackluster unit upon release but one of my very favorites as he was an absolute monster in Radiant Dawn.
The build I've got is extremely investment heavy so if you're looking for something quick, go somewhere else. If you're going all in on this and want to see him destroy everything, then you definitely have come to he right place.

That said, I wanna discuss a few things first. You'll notice his B skill is not what I'm talking about. Well, that's because gamepress apparently decided Wyvern Flight didnt deserve a place in selection. So I couldnt pick that for his build despite it being what I use.
Second, this build is also reliant on debuffing the opponent. You'll need a support like Mist, Male Morgan, Ashera, or Gunnthra who can reliably debuff the opponent so the visible stats cause the skill to be easier to activate.

With that in mind, Let's get into it.

Stats:
At a default 5☆ with no merges but his weapon refined and equipped his stats will look like this [HP 50, Atk 51, Spd 29, Def 40, Res 18]
After merges and dragon flowers, everything improves by +7 to a respectable [HP 57, Atk 58, Spd 36, Def 47, Res 25]
S supporting him will get him an extra 5 more hp and +2 to every stat.
[HP 62, Atk 60, Spd 38, Def 49, Res 27]

Weapon: Plegian Axe [+def]
This synergizes well with Wyvern Fight and helps make it so Haar deals disgusting amounts of damage while taking none himself. On a regular basis, because of this effect, the enemy regularly gets -19 atk and -19 def total during combat because of the combination of Plegian Axe and Wyvern Flight. It's insanely effective.

Assist: Reposition
While buffs are good, I commonly use reposition. Just my place style choice.

Special: Bonfire
Ignis's extra damage is unnecessary and takes too long to charge. Bonfire has proven to be far more reliable and with his defense default being 46, an extra 23 damage is usually more than enough to finish off opponents.

A Skill: Atk/Spd Catch 4
This synergizes with the debuffing build and gives Haar the necessary speed buff during combat to reliably not get doubled. An additional +9 atk and speed puts him at Atk 69/Spd 47 and the activation requirements for this skill are very easy to meet. This in addition to Spd/Res solo gets him +15 spd during combat at max activation so his speed at max is 53.

B skill: Wyvern Flight
Alright. Let's talk about this skill. The fact is, Haar will never be fast enough to activate a skill like Divebomb and he'll never be as fantastic a brave axe user as Cherche. With both Gerome and Beruka having refines as well, they have obtained something of a niche while Haar effectively has nothing. This skill though was made for semi fast but tanky fliers. Haar is exactly that.
Having a high defense of 49 thanks to refining plegian axe and a middling speed of 38 max even after a boon, it puts him in a very hard position to work with. Wyvern flights requirements demand his visible speed stat has to be greater than the opponents speed (-10) in order to activate the rest of the skill. Meaning as long as the opponent has 48 or lower visible speed Haar will get to compare their defense and debuff them.
HOWEVER visible buffs affect Wyvern Flight. This is where this gets nasty. Let's say you use Spd tactic to give him an additional 6 speed. Suddenly the enemy has to have 54 visible speed in order to not get hit with Wyvern Flights atk and def penalties and in a meta where in-combat buffs run rampant this isnt that hard to do. Even high speed units aside from the very most ridiculous will be subject to this. Summer Byleth, as an example, with a spd boon, max investment, and full flowers hits 52 spd. That's easily in Haar's range for Wyvern Flight. And if your debuffer is doing their job right, that means summer Byleth will be below 50 speed when Haar initiates (because if Haar gets attacked, he dies thanks to his low res).
Now if you're like me, you want numbers. So let's talk numbers.
Visible debuffs for atk will hit -7 max, Wyvern flight can dump another -7 during combat, and the Plegian axe will take the visible debuff and apply -12 during combat.
So what we have is visible (7) + in-combat (7+12) = total (26).
Yes, you read that right, atk will be debuffed by a total of 26 max. It's the exact same for defense.
This is huge for Haar because even with 27 res, I've had him tank more than his fair share of magic. Giving him res buffs just makes him even more ridiculous because his survivability sky rockets.
While each skill individually, they work extremely well together. Wyvern flight is a vital part of this build because of how well it works with the plegian axe. Speaking of which.

C skill: Def Smoke
As long as you get to pick the engagement which should be easy since Haar is a flier, smoke skills get immediately applied to everyone in his vicinity. And in Aether raids, this is generally everyone because of the way people cluster. Atk Smoke is just as good an option. I side with Defense smoke because of the high merges and how people generally work their defense up when it comes to armor units. Haar takes minimal damage as is after debuffing them so it's been slightly more consistent over atk smoke.
One thing to note here is that if you dont want to use smoke skills, Atk/Spd Rein is another phenomenal option. It instantly puts opponents max debuffs at
[Atk -30, def-26, spd -4] but you give up the assurance of debuffing multiple foes.

Seal: Whatever you want.
Personally, I side with Spd/Res Solo for a little extra damage and survivability though Atk/Def Solo, Atk/Res Solo, Def/Res Solo or a bracing or blow skill would work just as well. This comes down to simply whatever you think suits the build best. Just make sure you dont use bond skills because Haar has to be alone. Otherwise you'll negate Plegian Axe's effect from activating.

Pros:
In a meta like this where in combat buffs are effective people dont typically expect visible buffs. And while he may get debuffed his HP is high to avoid panics and, paired with 3 mythics in their seasons, he can avoid the panic manor in aether raids.

Taking at max -26 atk will cripple all but hard hitting mages. An opponent will typically need to special proc in order to kill him but generally he'll have already handled them before that as long as he can hit back.

Flier mobility means he can generally reach opponents before they can reach him so he'll typically get to engage on his terms.

Cons:
Beruka runs this build just as well as Haar does and she's far easier to obtain despite having less stats to work with (slightly less spd and way less atk)

Res targeting opponents and specials will absolutely obliterate Haar. Mages are an absolute must avoid unless you're engaging them.

He is fairly reliant on a debuffing partner to run beside on his team which will take up a slot. Ashera is a perfect partner for him in Aether Raids as she can essentially null-panic him and add +6 atk/res to him while she's at it.

S-Rank Summoner support is required.

This build is designed to do justice to Haar where IS failed, making him into a speedy player phase god. Dive bomb should be in the B slot (website not updated, had to use desperation). This Haar build can kill so many common threats in one round of engagement and still tank higher threat physical units. Worth the investment

[5*, no merges]
[HP/Atk/Spd/Def/Res = 50/51/25/40/18]

My default axe flier physical tank, Haar is able to take plenty of damage and dish out plenty in return. At no merges, his resistance is very low, so I focused on his defense instead.

Hack-o'-Lantern+ (+Def) gives the Guard ability without the HP threshold requirement, helping him deny opponent's specials.

Reposition is for moving other units out of the way. His default Swap works too, but I find myself using Reposition more often.

Bonfire, Sturdy Stance 2, Quick Riposte 3 are for helping him dish out damage of his own in retaliation.

Odd Def Wave 3, part of his default set, helps to boost his already high defense to even higher levels. On flier teams, I prefer using Goad/Ward Fliers.

Iote's Shield is for protecting him from arrows. With this and his high defense stat, he takes very little damage, if any, from most archers.

Yes, Haar's introduction to Heroes may have been a bit lackluster. But! With the right build, he can be quite fun to use. Sturdy Stance 2, and Odd Def Wave 3 play off Haar's already impressive Def. Hack-o'-Lantern is great for avoiding special activations, further adding to his hardiness. And Renewal to add to his survivability from Radiant Dawn.

Haar has an opportunity that all fliers haar capable of, that is running a strong and consistent bond build.

Faithful Axe+ with a speed refine will increase Haar's base speed from 25 to 28. Faithful Axe's effect will add +3 to Atk/Spd/Def/Res when he is adjacent to an ally, increasing his stats to Atk 57/Spd 31/Def 39/Res 21.

Introducing bond skills will enhance this even further. With Spd/Def Bond in his A skill slot and Atk/Spd Bond as his seal we push his stats to Atk 62/Spd 41/Def 44/Res 21. Now that Haar's speed has passed 40 he can double a lot of units without the aid of a guaranteed follow-up skill(Ex:Quick Riposte, Brash Assault).

To tie his role as a tank together, Reposition and Def/Spd Link will not only in keep units adjacent to him, but will have the added benefit of further increasing his Spd and Def by 6 making his peak stats(not including Drives/Spurs) Atk 62/Spd 47/Def 52/Res 21.

Finally, thanks to Guidance there will almost always be a unit adjacent to Haar and this will keep his bond skills consistently activated.