Yao - Nightglow

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.

Nightglow is a 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 as most of her buffs only apply during her ult, although those are usually minor drawbacks for such a strong ballistic support, although for very short fights where she can't use her ult Nightglow can potentially perform pretty poorly.
When used as a main DPS, Yao has low dps but very high DEF and only average mobility, and her damage scales based on her DEF rather than her ATK, which can make teambuilding with Nightglow more difficult than other operatives, so Yao isn't a very effective dps and should only be used as a support.

Overall

Role: Support - Ballistic
Deals Electric Damage
Weapon Type: SMG
Deals mostly skill damage
Mobility is mediocre

How to use Yao - Nightglow (as a support)

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

How to build Yao - Nightglow

Weapon: Starfall / Divine Justice
Weapon Parts: Max DEF
Logistics Squad: Zephyr
+10.0% DEF
+106 Alignment Index
+8.5% Electric Damage
Best teammates:
All operatives that deal Electric Ballistic damage
All operatives that deal Ballistic damage.

Specifics:

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 isn't very good as a main DPS you probably won't use her as one.

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 to you.
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 in most cases less than it sounds like, as Crit DMG Amplifier only buffs the extra damage from the crit, not the total damage of the crit hit. It also stacks additively.).
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 and also buffs allies ATK (how much ATK is 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, then if you use her ult 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. She also can't move while in that state.

The ATK buff of this skill is based on Nightglow's DEF, not her 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, and causing enemies to take extra ballistic damage.
As usual for 5* weapons, enemies taking extra damage is the strongest part.
Divine Justice
Her signature 4*, offering a nice DEF boost and ballistic damage buff during her ult

If you don't have either of her signature weapons, then consider:

Arctic Fox gives Alignment Index, which is useful as it boosts Yao's DEF, meaning it's generally Yao's best non-signature weapon.
You can also give her Frigatebird in order to give your main dps an ATK buff after using Nightglow's Support Skill, but since Frigatebird's ATK buff is based on Nightglow's ATK, and Nightglow has a low ATK stat, it won't result in a particularly large buff. A small buff is still a buff though.

For Weapon Parts Yao wants DEF.

Logistics

Use Zephyr squad - it was designed specifically for her and is her best choice.
Zephyr gives +25% CRIT DMG Amplifier on targets hit by her Spark Tether, and a +30% ATK Boost during her ult.
Zephyr also buffs Yao DEF by 24%, which is good for helping max her buffs.

For stats Nightglow wants:
Basic Stats: +10.0% DEF
+106 Alignment Index.
Special talent +8.5% Electric Damage

All of Nightglow's effects are based on her DEF, not her ATK, so you want as much DEF as you can, and Yao doesn't want ATK.
Alignment gives more DEF, so Alignment is her second best stat.
There's nothing else Nightglow can make much use of beyond DEF and Alignment anyway:
Yao can't heal
Yao doesn't create shields
She deals mostly skill damage, and skills can't crit, so Crit Rate and Crit DMG Amplifier aren't useful for her.

For her Special Talent Nightglow wants Electrical DMG, as she does deal Electrical DMG, and all of the other Special Stats aren't useful to her.
Yao doesn't deal much Electric damage, so getting Electric Damage on her logistics will only affect how she performs by a tiny amount.

Manifestation

M1 gives 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 for Nightglow, although it requires quite a bit of investment.
M2 converts an extra 20% of Nightglow's DEF into ATK and Max ATK during her ult (from 30% to 50%)
At 3,000 DEF, an extra 20% conversion would be an extra 600 ATK (from 900 to 1,500), which is a good ATK buff.
M3 increases the CRIT DMG Weakness / crossbow Ballistic DMG Bonus her spark tether gives by 20%.
+20% CRIT DMG Weakness is basically +20% CRIT DMG Amplifier.
This is a pretty decent bonus, but how useful it is depends on how much base crit dmg an operative has
If your operative doesn't crit much, or has a very low Crit DMG value, then this won't necessarily affect their damage much.
M4 gives Nightglow's ult +1 level.
This increases 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 for M4 buffs, the bonuses are around 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%.
This is quite a niche buff as many enemies just don't have shields, and hence M5 is both a relatively minor buff, but also a somewhat niche one.

If you want efficiency (good damage with minimal investment), then M3 is a good stopping point.
If you want Yao to support an operative that deals ballistic damage but who doesn't benefit from Crit buffs (e.g. Katya - Rhapsody) then you can generally stop at M2 instead just fine.

Good Teammates for Yao - NIghtglow

Yao's ATK buffs mean that she can buff 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, which are also generally the operatives that can make use of her crit buffs as well, although operatives who can deal crits with their skills can also make use of Yao's crit buffs too.

If you aren't also using Vidya - Celestial Swan alongside Nightglow, then Nightglow is a fantastic support for all ballistic-damage operatives.
Nightglow works well both for non-Electric damage ballistic-damage operatives that crit a lot and have high crit dmg values (e.g. Bubu - The Geomancer) but also for operatives that don't crit a lot but can still make great use of Nightglow's ATK and ballistic buffs (e.g. Katya - Rhapsody).

If you are using Vidya - Celestial Swan alongside Nightglow, then since Yao constantly deals Electric damage, your main DPS will need to deal Electrical Ballistic damage as well.
There aren't many such dps operatives, but all of them work well with Nightglow:

Chenxing - Jade Arc - Jade Arc deals mostly skill damage, but all of her damage benefits from ballistic buffs and always crit, so Nightglow is a perfect support for her.
Mia - Starchaser - All of Mia's standard rotation is ballistic damage and is guaranteed to crit with high crit damage, so Nightglow is a perfect support for her.
Chenxing - Ethereal Cloud - Chenxing deals lots of ballistic damage, and shoots weakspots a lot, and hits against weakspots always crit.
Fenny - Coronet - Coronet doesn't have a high Crit DMG value (30%) but she crits a lot, so Nightglow is still great for her.

For those operatives, Nightglow and Vidya - Celestial Swan are a fantastic pair of supports, and are generally their two best supports.