Rutger

Seal: Kestrel Stance 2

Using Windsweep without Null Follow-up might sound impractical, but it actually works quite well on Rutger. He has a very nice stat spread, which gives him good offensive power, great speed to avoid counters with windsweep (as well as Spd-based damage reduction), and just enough bulk to survive the initial enemy phase attack, finishing off with Vengeance.

This is a very high-risk high-reward build for Rutger. He uses Fury to boost his stats and damage himself, providing for a stronger Vengeance. Special Spiral with his Slaying Edge+ means that he’s using Vengeance in nearly every encounter which, with the boost from Fury, means that his damage output isn’t likely to be an issue. Rouse Spd/Def and his Steady Posture seal both provide him with some extra defense and help him net follow-ups, although Rouse Spd/Def could be replace with Rouse Atk/Spd if you have it available as a more offensive option. His Support skill is flexible for this build; I chose Reposition so that he could take hits for his allies and increase Vengeance’s power further. One could also use the +Atk refine on his Slaying Edge+, but I prefer to use +Res because his Res suffers otherwise.

This Rutger build has skills that are hard to get, so only use this build if you are really going to use him a lot.
For his weapon, it will be a Wo dao+ (+Spd) since this build is really reliant on his speed stat and he will be relying mainly on activating his special for damage. He will have a base 43 speed with a speed asset and with the wo dao it will be 46 unmerged. His weapon category is glitched so it wont let me choose Wo dao. If you cant give him a wo dao, use his slaying edge with a plus speed refine.
It doesnt matter what assist skill he has.
Luna for his special since his attack stat isnt that great, so itll halve the enemy defense stat and it goes well with the rest of the build.
Flashing blade 4 so that luna will be able to activate on Rutgers second hit and flashing blade 4 gives him +5 extra damage. It will only activate on the second hit against a melee unit. Or if you use his slaying edge itll activate on his second hit against any enemy if they dont have any of those reducing your special cooldown charge skills.
Spurn 3 is more recommended in my opinion, but it doesnt show spurn 3 on the B skill list so I had to click close call 3 or repel 3. Spurn 3 because it lowers how much damage he takes and also gives him +5 damage when his special activates if his under 75% health. Or if you dont have spurn 3, use closs call 3 or repel 3. If you have none of those and you are using the slaying edge, then use desperation 3 for an instant luna when under 75% health.
Atk/Spd Oath 3 for the extra attack and speed, unless you wanna go with any other oath, rouse, or even a wave skill depending if you wanna make him a solo unit. Wave skills are for those without oath or rouse skills.
Swift Sparrow 2 for the extra speed for activating all those speed skills, or phantom speed 3 if you want. I think his speed is already high enough to activate those skills.
I believe this build would be a mixed phase build, as the only thing he does not get during his enemy phase would be +4 attack and speed. With this build, he will be doing +15 or +20 guaranteed special damage each time if you use the wo dao build... i think. Im not good at making builds so im not sure if this is a good build or not.

Boon +SPD

This build is basically a better F2P double lion with no HP thresh hold.

Weapon: Brave Sword/ Brave Sword+/Wo Dao+
Rutger can quad almost the entire hero roster with this minus super speedy units like F!Ike, Mareeta, Larcei etc. If you have a S!Byleth, you can pair the desperation effect for a straight quad attack with no pause. And if moonbow is ready before initiating combat, you can trigger it 2 times on the 1st and 3rd hit after combat moonbow will be ready to get danced and another round of combat. Or Wo Dao can be used for more damage.

A slot: Life and Death 4 this is a flat damage and speed boost letting him reach 51 ATK adn 50 SPD with 10 dragonflowers. With full merges, Rutger can quad a fully merged spd build for F!Ike if he has Joint Hone Spd active.

B slot: Any spd check damage reduction skill, best one to use is Spurn 3 for a guaranteed 10 damage if all conditions are met with moonbow. These use Rutger's offensive stats for more survivability and tankiness.

C slot: Atk/Spd Oath 3 is a flat SPD and damage boost that can work every player phase as long as Rutger is next to someone like a dancer for example or Annette.

Sacred Seal: Flashing Blade 3 you're gonna need this for moonbow to proc twice during one round of combat. Rutger will outspeed most of the roster any way and if not he can still trigger moonbow but only once.

This build attempts to capitalize off of Rutger's impressive speed, and buff his lackluster attack stat. The Cake Cutter acts as Swift Sparrow, increasing attack and speed in the player phase. Fury 4 allows him to increase all stats during combat, and dealing 8 damage to him after a combat interaction. This allows him to essentially have controlled access to his B skill, Desperation 3, which guarantees that he will counterattack before the opponent when under 75% health. His C skill Atk/Spd Oath gives him a +5 boost to attack and speed if adjacent to an ally unit, working well with buff units or dancers. Overall this build gives him a terrifying increase to speed and a quite necessary buff to damage, coming out to a possible maximum of 52 attack and 59 speed without merges, and counterattack priority. If paired with a buff unit these stats can be increased further, and if paired with a dancer he can move across the map at high speeds possibly netting 3 kills per turn.