Young Marth

Preferred IVs: Atk+ or Res+
• Baby Marth’s attack even without a boon is already pretty impressive at 36. However, every point in attack more means more damage against cavalry or armoured foes which therefore increases the likelihood of an one-shot. A boon in resistance is also a good choice, considering that Baby Marth has a super boon in resistance, getting his res up to 30. A boon in resistance is beneficial for any Distant Counter builds so they can survive a blow from a tome. Plus, with the Rapier granting Baby Marth Vantage you would want to get him into Vantage range as safely as possible. The issue with a res boon however is that Marth would need to sacrifice points in other stats which he however all needs, since f. ex. a speed bane would reduce his likelihood to double enemies and would increase the damage reduction from skills like Repel 3 while a defence bane would make him more susceptible to physical damage.
Weapon: Rapier
• Baby Marth comes with an absurdly good personal weapon, being the Rapier. The Rapier is a 16 might sword with dual effectiveness, Vantage and on top of that it also grants a flat +3 bonus to speed. Did I mention that Marth is a 4-star hero? You wouldn‘t guess it from his kit. With that weapon Marth is in an odd position, since this weapon has both defensive and offensive synergy. I personally however believe, that Marth is best put on the defences and can function pretty well in Aether Raids defence.
Assist: Ardent Sacrifice
• His Assist matters little. Just slap something onto him that has defensive synergy like a Ardent Sacrifice or a Rally. Preferably he would like to get into Vantage range safely and Ardent Sacrifice or Reciprocal Aid can help with that.
Special: Hero‘s Blood
• On top of having an amazing (almost) personal weapon, Marth also comes with an unique special that is the exact same as Legendary Marth’s and Christmas Marth’s Fire Emblem special. There is realistically no reason as to why you wouldn’t want to use Hero’s Blood, since it increases damage based on speed which is Baby Marth’s highest stat. It also is useful in combination with Vantage, since it will be ready after your first encounter if neither Marth nor the opponent attacked twice, so it can be used on the enemy phase to Vantage kill an opponent. Also, in Infantry Pulse set ups, you will only need two units with Infantry Pulse to fully charge the special which frees up the C-Slot on other units on your Aether Raids defence team.
A-Slot: Distant Counter
• TIME FOR SOME CHEESE! Vantage on the weapon plus dual effective damage is a natural fit with Distant Counter. The idea is that your Baby Marth is reduced below 75% HP on player phase so that he can murder cavaliers and armoured units before they can to anything to him. Considerably worse units than Marth with this very build are in the right scenario capable of soloing entire maps on their own. However, these units can usually be stopped right in their tracks by either the Reinhardt or Brave Veronica (a. i. strong, offensive healer with the Razzle-Dazzle combination). While Marth can do fairly little against the Reinhardt since even a maxed out, res+ Marth with this build cannot survive a decently invested Reinhardt if Marth is at full HP. However, there is something he can do about the Razzle-Dazzle healers…

B-Slot: Null-C Disrupt 3
• The combination of Vantage+Distant Counter+Null C-Disrupt alone would be broken enough, but Marth adds dual effectiveness on top of that and becomes hands down the best user of the Distant Counter + Vantage combination in the game. Because every other choice either is hard to obtain (Phina) or lacks Null-C Disrupt (Young Caeda and the Distant Counter weapons users). It’s fairly likely that a maximum investment Baby Marth could realistically solo even difficult maps like Legendary Hero Infernal Maps under the right circumstances. Because with this build the only conceivable way of safely killing Baby Marth is Hardy Bearing which isn’t the most common Seal in the world. That also greatly helps Marth’s survivability since its fairly unlikely that he has to eat a hit.
C-Slot: Variable
• At the end of the day his C-Slot is pretty unimportant. There are of course useful skills like Savage Blow 3, Times Pulse 3, Odd/Even Atk. Wave 3, Panic or Defence Smoke 3, but no matter what you pick, it won’t influence his overall performance all that much.
Sacred Seal Slot: Brazen Atk/Spd 3, Brazen Atk/Res 3 or Brazen Atk/Def 3
• Which one you pick is up to preference, but the idea is always the same: More damage. With an additional +7 attack once he drops below 80% HP Baby Marth is almost guaranteed to one-shot any cavalier or armoured knight without their respective protective shields.

Issues with this build:
• 1. Loses to prominent members of Aether Raids offense: This Baby Marth build belongs in Aether Raids defence teams in my opinion, however Aether Raids offense is filled with a lot of units Marth just straight up loses against. While Reinhardt has been less prominent as of late, he still pops up every now and again and if Marth is not in Vantage range, Reinhardt will just kill him before he even has the chance to do anything. Ophelia will also one-shot him, if she has Hardy Bearing equipped. Other prominent blue picks also spell trouble, like Brunya, New Year’s Alfonse or Legendary Chrom. However, a smart defensive set-up can mitigate this issues somewhat.
• 2. Fairly weak at full HP: The most prominent weakness of Vantage builds is the fact that to function at all, the unit needs to be low on HP to function. Marth is fairly bulky and against most enemies he will get into Vantage range somewhat safely, but it’s no guarantee and the AI in Aether Raids defence is not smart enough to utilize Ardent Sacrifice or Reciprocal Aid in any meaningful way.
• 3. Very high investment cost: While Distant Counter has become available for free-to-play players through the Compile Manual feature and Male-Byleth who was distributed to celebrate the release of Three Houses, it is still rare and hard to get. And Null-C Disrupt is not yet available through any free-to-play friendly means and is only on Nailah, a five-star exclusive unit that did not have many appearances on banners. I mean, I summoned three Nailahs in my time, but there is a very likely chance, that you will never even see a Nailah.
• 4. Limited-time unit: The „The start of it all“ banner is a limited-time banner, meaning that you only have the opportunity to get Marth in that very small window of time and after that it may take months until Baby Marth reappears on another banner. So, unless you throw orbs at this banner like there’s no tomorrow, it’s somewhat unlikely that you will get enough Baby Marths to plus ten him.

Young Marth appears to be underpowered compared to Young Caeda, but in reality, he's wayyy better than her. Marth is available as a 4*, which makes it fairly easy to summon him. Rapier is a great weapon, and was previously exclusive to Phina. Reposition. If you're pulling on reds, chances are that Caeda might show up too. This is great, as she can be used to give Marth distant counter if you don't want to keep her. Special spiral is a great choice for his B slot, as it allows Young Marth to proc vantage and Hero's blood at the same time. C slot is flexible, spd/def oath is a great skill that comes with his base kit, smoke skills work well too. For the seal, any brazen skill will do, but brazen atk/spd works best because Hero's blood adds damage based on spd.