Ylgr

Weapon: Sylgr (+EFF)

Assist: Positional Assist / Flexible

Special: Moonbow / Ruptured Sky

Passive A: B Duel Infantry / HP/Spd 2 / HP/Atk 2

Passive B: Even Pulse Tie / Ruse Skills / Chill Skills / Link Skills / Sudden Panic

Passive C: Spur Skills / Joint Drive Skills / Spd/Def Oath / Rouse Skills / Flexible

Sacred Seal: Drive Skills / Spur Skills / Rouse Skills / Atk/Res Oath / HP/Spd 2

• Ylgr’s surprisingly high range and ceiling in applying her refined PRF’s debuffs (as proper skill usage means she can apply up to -7 Atk/Spd/Def/Res on foes pretty consistently) makes her a standout support unit, especially in the context of Savior armored teams given how easily proximity may be maintained without consequence. 
• This build essentially aims to develop this supportive quality by stacking further support tools, most notably by enhancing her HP as much as possible. This is beneficial for Ylgr’s own performance in the context of Aether Raids (and Colosseum modes to an extent) by helping avoid the Panic Tower and other sources of HP-based Panic, as this inhibits her utility if using buffs to leverage Sylgr. 
• Moreover, this enables the usage of powerful HP-based support skills, like Even Pulse Tie to neutralize threats wielding instant Special activations and Sudden Panic to debilitate teams that rely on visible buffs. Ylgr does have other B skill options like Ruse, Chill, and Link skills for more consistency in activating her support — but at least where Even Pulse Tie is concerned, they're generally not as potent.
• For both her C and Sacred Seal slots, Ylgr has two main overarching options to consider: she can either utilize in-combat buffing abilities like Spur Atk/Def and Drive Atk to contribute even more to the effective stat swings she can provide her team, or she can equip self-buffing skills like Oaths and Rouse Atk/Spd to enable Sylgr without relying on outside assistance. Interestingly enough, Ylgr can use Spd/Def Oath and Atk/Res Oath as her C skill and Sacred Seal slot respectively to guarantee a Spectrum -5 on nearby foes by simply sticking close to her team (which she already wants to be doing).
• Ylgr’s Special is likely the least important aspect of her build here as she should be seeing combat fairly infrequently, rendering Moonbow the best option in most cases as its low cooldown makes it quickly accessible in a pinch. For scoring purposes (and slightly more specialized combat capacity), Ruptured Sky may be considered.

Weapon: Sylgr (+EFF)

Assist: Positional Assist / Flexible

Special: Moonbow / Glimmer / Luna 

Passive A: Swift Sparrow / Atk/Spd Solo / Atk/Spd Push / Sturdy Impact

Passive B: Desperation / Null Follow-Up / Spd/Def Tempo

Passive C: Atk Smoke / Spd Smoke / Joint Drive Skills / Spur Skills / Spd/Def Oath / Rouse Skills / Flexible

Sacred Seal: Swift Sparrow / Flashing Blade / Atk/Spd Solo / Atk/Spd Form / Rouse Skills 

• Alternatively, Ylgr can opt to lean more towards the offensive perks of refined Sylgr, as +9/+12 Atk/Spd and Speed-based bonus damage is honestly nothing to scoff at. This is not to mention the fact that she can leverage her own in-combat debuffs to fortify her battling consistency — and moreover, these stat swings are always available for her allies’ gain when necessary. Both +ATK and +SPD IVs work well in this context. 
• Glimmer and Moonbow are among Ylgr’s best Special candidates; their low initial cooldowns allow for consistent triggers in most rounds of combat and pair nicely with the Flashing Blade Sacred Seal when using Desperation. Luna can also be considered for the stronger burst it provides, though it does activate more infrequently. 
• Since she relies on both Atk and Spd for her damage output, any A skill that enhances them simultaneously makes for a natural complement to her playstyle; Swift Sparrow, Atk/Spd Solo, and Atk/Spd Push are among her best options. Sturdy Impact may also be worth considering despite its lack of Spd enhancement, as the +10 Def and follow-up negation it brings greatly alleviates her low physical bulk so she can more safely run B skills that aren’t Desperation.
• Speaking of, Desperation tends to be Ylgr’s prime B skill for the simple fact that her poor overall bulk — both physical and magical — makes it difficult to engage in continued engagements otherwise. However, should you feel confident in her longevity (particularly if able to consistently maximize the in-combat Atk debuff she applies), other potent skills include Null Follow-Up to bypass common enemy mechanics that inhibit traditional nuking and Spd/Def Tempo to ensure stronger Special activations.
• In the same vein that self-buffing skills work markedly well here as she is in a position to leverage their strengths beyond supportive purposes, Atk and Spd Smoke become more attractive options as Ylgr will actually be expected to constantly initiate combat to apply their effects, which may then be doubled in effectiveness via Sylgr. Joint Drive and Spur skills are also worthwhile if more support is desired.