Yao - Nightglow

Yao - Nightglow is not currently available as a Trial Operative in any story mission, so if you don't have her the only way to try her is to wait for her banner so she has a trial.

Yao - Nightglow is a fantastic support operative who buffs ally ATK and ballistic damage, whilst causing enemies to take more crit dmg, and her buffs have a long duration, although Nightglow is much better for ballistic operatives than skill dmg operatives, and the high U-Energy cost of her ult [60 U-Energy] means she can take a while to get going, although those are minor drawbacks for such a strong ballistic support.

Overall

Role: Support - Ballistic
Deals Electric Damage
Deals skill damage
Mobility is pretty terrible

Advantages

Provides strong ATK, ballistic, and crit dmg buffs
Very good uptime on her buffs
Extremely high DEF (3,000+)

Disadvantages

Not a great support for skill dmg operatives
Scales entirely on DEF, not ATK, which is unusual
(and makes buffing her a bit harder)

How to use Yao - Nightglow

1. If Nightglow has 60 or more U-Energy, use her ult
2. Otherwise, spam her support skill off-cooldown

Note that all of Nightglow's effects for all of her skills scale off of her DEF, and absolutely nothing is based on her ATK.

Standard Skill
Yao launches two bombs that fly towards nearby targets.
Each of these also restore 1.5 U-Energy.|
As this skill has a 4s cooldown, the U-Energy gain Yao can get from this is actually decent, although since Yao is a support operative who is pretty terrible as a main DPS she won't ever want to be a main DPS, so she'll never use her standard skill.

The damage of this skill is based on Nightglow's DEF, not ATK.

Support Skill
Yao creates a Spark Tether, which deals electrical dmg to up to 3 enemies each second, whilst also restoring 1 U-Energy.
The targets hit are also inflected with CRIT DMG Weakness, meaning means that crits against them have increased CRIT DMG Amplifier - which means more damage (but less than it sounds like).
Since crossbows can't crit, they instead get extra ballistic dmg.

The damage of this skill is based on Nightglow's DEF, not ATK.

Ultimate Skill
Basically a souped-up version of her support skill, except it can target 6 enemies, also buffs allies ATK (based on Yao's DEF).
This consumes 2 U-Energy each second, and prevents U-Energy regen. It also has a max duration of 30s, so you can just use it whenever you get to 60 U-Energy.

The max ATK bonus Nightglow gives is based on her DEF.
To max the buff needs 5,000 DEF, or 3,00 DEF if Nightglow has M2.
5,000 isn't feasible, but 3,000 DEF is possible

If you're in multiplayer or some other content where Nightglow is your only operative, you get to fly in the air and watch as Yao deals damage from the skies.
Whilst it's by-far the easiest way of playing Snowbreak by a huge margin, Nightglow on her own isn't very effective.

The ATK buff of this skill is based on Nightglow's DEF, not ATK.
The damage is also based on her DEF.

Weapon

Starfall
Her 5*, and her best weapon., offering a nice DEF boost, extra ballistic damage from her ult, causing enemies to take extra ballistic damage
Divine Justice
Her signature 4*, offering a nice DEF boost and ballistic damage buff during her ult

For Weapon Parts just go max DEF.

Logistics

Use Ithira squad - it was designed specifically for her and is her best choice.

For stats you want +10.0% DEF and +106 Alignment Index.
For the Special talent, you want 8.5% Electrical Damage

All of Nightglow's effects are based on her DEF, not her ATK, so you want DEF.
Alignment gives more DEF, so Alignment is good.
There's nothing else Nightglow can make much use of beyond DEF and Alignment anyway.

For her Special Talent Nightglow wants Electrical DMG, as she does deal Electrical DMG.

Manifestation

M1gives allies a ballistic damage buff during her ult.
The buff is based on Nightglow's DEF, up to 15% at 3,000 DEF.
If Nightglow has at least 3,000 DEF during her ult, it also gives allies CC resistance.
3,000 DEF is possible, although it requires quite a bit of investment.
M2 gives an extra 20% ATK during her ult (from 30% to 50%)
The max bonus of 1,500 remains unchanged, but with M2 Nightglow can feasibly get the max bonus.
M3 increases the CRIT DMG Weakness / crossbow Ballistic DMG Bonus her spark tether gives by 20%.
M4 gives Nightglow's ult +1 level, as well as increasing the percent of her DEF that gets converted to ATK, increasing the Spark Tether DMG, the CRIT DMG Weakness and crossbow Ballistic DMG Bonus.
As usual, the bonuses are 10%.
Since Yao can max the 1,500 ATK bonus of her ult with just M2, the DEF->ATK conversion boost is basically worthless here, although the other boosts are very nice.
M5 increases the amount of damage ballistic shots do to shields by 10%.

If you want efficiency (good damage with minimal investment), then M3 is a good stopping point.

Good Teammates with Yao - NIghtglow

Yao's ATK buffs mean that she can buff with basically all Main DPS-ers to at least some degree, however Yao gives a lot of ballistic buffs and hence works best with operatives who deal ballistic damage.

If you are using Celestial Swan alongside Nightglow, then your main DPS will need to deal Electrical Ballistic damage.
There aren't many such operatives: Jade Arc, Ethereal Cloud, Wednesday, Coronet, and The Observer.
For those operatives Celestial Swan + Nightglow is a fantastic pair of supports.