Summer Byleth (F)

Weapon: Sun's Percussors

Assist: Reposition / Draw Back

Special: Glimmer / Ruptured Sky

Passive A: Life and Death (3 or 4) / Fury (3 or 4) / Atk/Spd Push 4

Passive B: Windsweep / Chill Spd/Res 2 / Watersweep

Passive C: Ground Orders / Goad Fliers / Flexible

Sacred Seal: Atk/Spd 2 / Chill Spd / Swift Sparrow

• Byleth/Rhea's unique weapon, Sun's Percussors, brings quite a few shiny things to speak of to the table when it comes to the role of an offensive tome flier—a niche that is already quite a great in itself thanks to a dominant attacking range.
  • It offers permanently reduced cooldown—a relative rarity among tome units—meaning shorter Specials such as Glimmer and Ruptured Sky can fire off with top frequency without assistance from Blade skills.
    • For general purposes, Glimmer typically has more damage potential due to the popularity of Resistance-weak units.
  • Additionally, if Byleth/Rhea's visible Speed is higher than the opponent's (in which case they prefer A slots such as Fury or Life and Death over their native Atk/Spd Push), or if their target is at 100% HP, they also gain:
    • A hefty swing of +5 Atk/Spd, improving their already great offensive game.
    • Built-in Null Follow-Up, allowing the duo to bypass obstacles that would otherwise hinder their speedy hits, especially Wary Fighter, Zephyr Breath, and especially their own Sweep skills.
• The rest of the build is pretty standard offensive mage fare, though Byleth/Rhea's ability to use Desperation at the press of a button gives them a couple of particularly fun B slot options to consider:
  • Windsweep yet again worms its way into a Byleth build, and for a similar reason to Creator Sword: it enables the duo to attack a group of slower units (all physically-inclined weapons) of your choice for basically free. Thanks to the Null Follow-Up effect of their weapon, they still maintain their follow-up attack as well! Watersweep can be considered for unhindered attacks against magically-inclined enemies. However, Windsweep is typically more useful as the majority of threatening Distant Counter units are affected by it and those typically pose more of a threat. No matter which one you pick, keep in mind that Byleth/Rhea can simply activate Desperation to beat out the other unit group.
  • Chill Spd/Res comes by default on the duo and passively debuffs an enemy specializing in those two stats. If it hits the right unit, it serves to make Sun Percussor's Speed comparisons more comfortable while also increasing Byleth/Rhea's damage.
• As an important reiteration, two of Sun's Percussor's effects locked behind either of two conditions: Byleth's visible Speed must be greater than her opponent's OR their opponent is at full health. Fulfilling the latter condition is easy enough, though it's best to make sure both are fulfilled at any time. Perplexing as it may be to pass up otherwise great skills seals and skills such as Swift Sparrow and Byleth's native Atk/Spd Push, it is generally safer to run out-of-combat stat swing skills such as Fury, Life and Death, and the Atk/Spd 2 or Chill Spd seal. 

Weapon: Sun's Percussors

Assist: Reposition / Rally / No Assist

Special: Ruptured Sky

Passive A: Sturdy Impact / Mirror Impact

Passive B: Aerobatics / Windsweep

Passive C: Ground Orders / Air Orders

Sacred Seal: Sturdy Blow / Hardy Bearing / Swift Sparrow / Atk/Spd 2

• Byleth/Rhea is also a rather decent choice as an Aether Keep defender for a few reasons:
  • Their status as a Duo hero unlocks the Duo's Hinderace structure, allowing your setup to impede Offense strategies and carry units reliant on activating Duo skills.
  • Their default access to Ground Orders and Flier status in general opens up a considerable relevant team options, including access to Impact skills and movement tech for your other defender units.
  • Byleth/Rhea's existing general offensive might (thanks to great stats and weapon) allow them to be a relevant threat most of the time—although not being affected by Infantry Pulse may hurt their contributions somewhat.
• A build focused on Aether Raids Defense, of course, requires a few specific options to account for AI control and general map coverage:
  • Impact skills are a bit pricey, but nearly non-negotiable for specialized Defense performance. They significantly improve Byleth/Rhea's player phase survivability by disabling their target's counterattack. In most cases, they can live to attack multiple times more.
  • Aerobatics (or Flier Guidance if you're using a flier-focused setup) increases Byleth/Rhea's range of attack and generally makes her movement somewhat difficult to predict for most Offense players. Alternatively, Windsweep allows for unhindered and uninterrupted damage against most Offense carry units, many of which are melee weapon users, not carrying Null C-Disrupt.
  • Hardy Bearing is a simple but effective countermeasure against Vantage users as well as Brave Ike's Urvan effects. Those concerned with stat performance or simply want to use Hardy Bearing on someone else may use a relevant Blow skill or Atk/Spd 2.