Yao - Winter Solstice

If you don't have Yao - Winter Solstice yet but want to try her out, she is available as a Trial Operative in Story Mission 9-12

Yao used to reign as both the games queen of dps as well as the peak of its... peakness, but it has been a long time since Yao was at the top. Despite slacking off the current dps charts Winter Solstice is still very competent when built and used properly, although her absolute lack of AoE and slow RoF make her slow to clear groups with, making her a relatively poor choice for general non-endgame content these days. Against bosses though she's still very capable, but no longer at the top.

The Basics

Role: Main DPS
Deals Thermal Damage
Deals entirely Ballistic damage
Mobility is very poor

How to use Yao - Winter Solstice

If her ult is ready, use it and shoot all its shots.
If Yao has her M1, use her standard skill to reload shots for her ult as often as you can.
Otherwise, use her standard skill and shoot the special shot if gives at enemy weakspots
Otherwise, just ADS and shoot enemy weakspots

How to build Yao - Winter Solstice

Weapon: Space Cowboy / Site of Racket
Chocolate Filling is also good
Weapon Parts: Max ATK
Logistics Squad: Amano-Iwato
+10.0% ATK
+10.6% Crit Dmg Amplifier
+8.5% Thermal Damage
Best teammates:
Vidya - Celestial Swan
Yao - Nightglow
(but not both at the same time)

Passive Effect

Passive Effect
Yao deals more damage the farther away she is from the enemy.

Yao passively has "Solar Armor" which gives 20% All-Type Resistance.
Since Yao doesn't move much, this is very useful for her survivability.

Standard Skill
Yao's standard skill causes her to chamber 1 special round.
If Yao's magazine is full, the special round will overwrite 1 standard round.
That special round deals AoE thermal damage.

With its first Neuronic, it deals extra thermal damage.
With its second Neuronic, it applies a 3s burn to targets hit, dealing damage every 0.5s.

This skill has a 13s cooldown, but can stack up to 5 charges.

This skill has an S-Eenergy cost of 8. Since this is so much lower than its cooldown, Yao doesn't run into any issues with S-Energy.

Ultimate Skill
Yao temporarily removes her Solar Armor to enter "Solar Flare" state which enhances her weapon by giving it a special magazine of 5 rounds.
Shots from the enhanced magazine deal much more damage, but don't grant U-Energy. They also have a slight charge-up time.
Yao can't reload these special shots.

You exits the special state and returns to normal when she reloads, runs out of ammo, or swaps out.

This has quite a short cooldown of 20s, but requires the default max of 100 U-Energy.
The huge U-Energy requirement means that Yao's ultimate can't be used very often.

Support Skill
Yao deals PBAoE thermal damage and knocks enemies into the air.
The places a trap that lasts for 5s, dealing DoT thermal damage.

With its first Neuronic, when the trap ends it deals extra Thermal damage.
With its second Neuronic, using a skill inside the trap consumes it and grants S-Energy or U-Energy.

Like all Main-dps operatives, Yao's support skill isn't very good, and you don't want to use her as a support operative.

Weapon

Space Cowboy
Her 5* weapon, and her best weapon.
Increases thermal damage, which is nice.
Gives a huge ballistic damage for the next shot after you reload. Her standard skill does proc this, giving her standard skills shots a huge damage buff.
Chocolate Filling
The 5* weapon from Paradoxical Labyrinth.
Chocolate Filling deals Chaos damage, which is not ideal, but not a major reduction to Yao's damage.
Given that Chocolat Filling does high damage, it's an excellent choice for Yao if she doesn't have Space Cowboy yet.
Chocolate Filling does extra chaos damage per shot every 5s, which Yao makes good use of due to her low RoF.
Site of Racket
This gives S-Energy when hitting a weakspot, improving her sustain of her standard skill.

There are other good non-Thermal weapons like Crystal Drill, Wetland Park, Mark of Mesmer, Sweet Soul, which are all fine, but since Chocolate Filling is strong and isn't too difficult to get you may as well use that.

For weapon Parts Yao wants max ATK.

Logistics

Use Amano-Iwato squad.
Gives extra ADS damage after landing a crit.
Considering all Yao does is ADS damage, the buffs are nice, and since she shoots weakspots so much and weakspots always crit, Yao crits a lot, building Amano-Iwato stacks very quickly.

For stats Yao want 10.0% ATK and +10.6% Crit Damage Amplifier
For the Special talent, you want 8.5% Thermal Damage

Alignment Index boosts Yao's long-range damage.
Alignment Index isn't as good as Crit Dmg Amplifier, but it's a good cope option.

U-Energy recovery is nice due to how strong Yao's ult is. It's not the best option, but it's an ok cope stat.

Manifestation

M1 allows Yao to use her standard skill to reload her ultimate.
Considering how strong her ult is, this allows Yao to save her standard skill charges to use them for her ult, allowing her ult to deal a lot more damage.
M2 causes Yao to refill all her standard skill charges when she uses her ult.
This means that Yao doesn't need to save her standard skill charges for her ult as you would want with just her M1 and not her M2.
M2 also reduces the cooldown by 3s.
M3 causes Yao to recover 15% U-Energy for each special round she has when her ult ends.
Theoretically, this can allow Yao to recover 75% of the U-Energy she used if she can use the M1 buff to keep herself at maximum rounds.
Even if you don't recover all 75% of her U-Energy, recovering some is still very nice, and will increase the frequency at which you can use Yao's ult a lot.
M4 gives Yao's standard skill +1 level, and reducing its cooldown to 8s.
Since her standard skill costs 8 S-Energy, its cooldown being reduced to 8s still doesn't cause her any S-Energy issues.
M5 causes Yao's ult to deal +2% Final DMG per shot left in the magazine.
Since it has a 5 shot magazine, this gives +10% Final DMG when the magazine is full.
Since Yao can recover shots with her standard skill, Yao can sustain the max +10% boost for many shots, making M5 strong.

As Winter Solstice is one the standard/permanent banner, you'll max all her Manifestations eventually anyway.

Good Supports for Yao - Winter Solstice

Yao deals entirely thermal ballistic damage.

Vidya - Celestial Swan (and other resistance shredders)
Celestial Swan and other resistance shredders (Acacia - Kaguya, etc.) that reduce the enemies resistance help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off element to Yao.
Yao - Nightglow
Yao can make full use of Nightglow's ATK, Ballistic, and crit buffs, and Nightglow is a fantastic support for Yao.
Note that Nightglow is off-element to Yao (Nightglow deals Electrical damage)
Eatchel - The Cub
A Blitzing Fangs Eatchel provides healing as well as a decent ATK buff.
Not the strongest support, but still good.
Mauxir - Shadow Ka
Yao really likes the utility of not having to aim, and the extra damage is nice.
Note that Shadow Ka is off-element to Yao (Shadow Ka deals Chaos damage)

Yao's low RoF mean that per-shot buffs like Tess - Phantasia and Fenny - Little Sunshine don't add much to Yao.

Fenny - Starshine isn't a great support for Yao, as whilst Fenny's support skill does work with Yao and Fenny's ballistic buffs benefit her, Yao reloads a round into her standard magazine, but rounds are taken from Fenny's special magazine first, meaning that Yao can't fire shots from her standard skill whilst Fenny's special clip is active.
This means that you can either use Fenny's support skill or use the shots from Yao's standard skill, but not both.
Similarly, since Fennys support skill and Yao's ultimate skill both give her a special magazine, they can't be used at the same time, meaning that Yao can't gain the benefits of Fenny's special magazine and its associated buffs whilst Yao is shooting her ult.
Overall, this makes Fenny a poor support for Yao.