Berkut

Seal: (personal preference)

Berkut is criminally underrated within the english meta. 34 ATK is nothing to scoff at, and paired with Death Blow, an effective 48 ATK puts him on par with most typical Brave users in the game. His poor SPD means he doesn't miss the extra 5 points taken from the Brave Lance+, either.

Berkut's good offense and excellent bulk makes him fairly unique among typical Brave users: Berkut can bait red and blue melee units. Lancebreaker guarantees him Bonfire on any lance matchup if he doesn't kill them before it activates, meaning he's sure to kill almost every lance user he's put against, and his excellent bulk mean's he'll survive any lance encounter he doesn't win.

Of course, the Brave Lance+ plus his good offense means he tears through every red and blue mage he initiates combat with, and a good chunk of the green mages too. His decent RES coupled with his high HP means he'll also survive hits from mages who don't double him (so, mages who are carrying Windsweep or Watersweep).

His C skill honestly boils down to the needs of the team he's on, but Threaten Def is a solid option due to his ability to bait and high mobility.

The Berkut I use on calvary teams and as my support unit. He has goad in order to support his offensive teammates where he receives hone fortify and ward from his teammates. He also has the hp+3 seal.

Neutral-

Life and death try's to help with his terrible speed, using desperation can actually make quad units and fortify cavalry just helps bulking some other units up.

So I know a lot of the community hated on IS and Berkut for not being on par with the top tier Horse Units and not have his signature weapon from SOV but having started a little late, I don't have access to those Horse units so I decided to make Berkut as good as possible so this is the set I came up with to make him good. With this set he is able to ORKO all red units (except Draug and Zephiel but can do that after a Hone Cavalry), all blue units (except Effie, Corrin (F) and Nowi but after a Hone Cavalry buff Corrin dies, Nowi lives with 1 and Effie lives with 6), all colourless units and all green mages except Boey. Most of the green cast still manages to KO him on retaliation.

Effie on wheels.
The idea behind the build is to capitalize on Berkut's naturally high attack and bulk.
Brave lance+ is the bread and butter allowing for the double attacks on initiation. Obviously the downside to brave lance is the speed loss, but Berkut's speed is already so low that it hardly makes a difference, which is perfect for this weapon.
Outside this, fury is too good of a skill. It helps increase his overall already good bulk and attack. The 6 damage is mitigable due to taking less damage from the increased bulk and can also be restored through renewal.
A slot can always be changed out for death blow to give more oomph to his attack, but i like having him a little more well-rounded with fury.
B slot is completely optional. I went renewal to help with losing HP to fury and it also generally goes well with him due to his bulk.
You could go with lancebreaker instead to have Berkut come out more favorable in those matchups. Swordbreaker isn't recommended as he already does well against most sword users.
As for his C skill, like most builds it's team dependent. I went with threaten atk to further increase Berkut's ability to tank physical attacks. Threaten def could be used here to increase one shot chances instead.
Outside this, if you're using him on a horse emblem team you could drop any needed cavalry buff on him. Or else just use a standard spur or fortify C.

I really like Berkut. Although he's outclassed by many other units, I feel with the right amount of love and devotion you can make him work wonders.