Yao - Quiet Quitter (a.k.a. "QQ")

If you don't have Yao - Quiet Quitter yet but want to try her out, she is available as a Trial Operative in Story Missions 6-8, 6-9, and 6-10 in story chapter 6 "In Pursuit of What".

Whilst QQ is basically useless as a DPS, she is a great healer.
QQ's healing is weaker than those from Eatchel - The Cub. QQ can use a specific support weapon to provide a decent ATK buff, although also weaker than Eatchel's ATK buffs.
Whilst QQ is the games second best healer, you generally don't need healers at endgame, or two healers at all, so QQ is generally only useful until you get Eatchel.

Overall

Role: Healer
Deals Frost Damage
Weapon Type: Sniper Rifle
Deals entirely ballistic damage
Mobility:
Herself: Average
Her Drone: Good

How to use Yao - Quiet Quitter

Add her to your team, and:

If Yao has a support-based weapon or logistics set, then spam her support skill off-cooldown
Otherwise, use QQ's support skill whenever you need healing
(and stand in its AoE)

How to build Yao - Quiet Quitter

Weapon: Deep Sea's Call
Weapon Parts: Max ATK

Logistics Squad: Moira
+10% ATK
+106 Alignment Index / +19.3 Skill Haste
+10.6% Auxiliary Unit Strength
Best squadmates for Yao:
Yao is a good healer for all operatives.

Specifics:

Standard Skill

Yao deploys a "Slackers Aid" for 10s, which
1. Continuously replenishes her ammunition
2. Adds Frost damage to each of her shots
3. Increases her Rate of Fire by 25%
This is actually quite a strong offensive buff, but it's also the only offensive thing that Yao has, so it still doesn't make her a good dps, and Yao is much better off being an off-field healer than a dps.

Support Skill

Yao deploys a drone that provides PBAoE healing every 3 seconds.
If Yao has a Frost weapon, then the drone lasts 30s on a 30s cooldown, giving it 100% uptime.

The healing from this drone is significant, and it doesn't take that much investment to get it to over 1,000 healing per pulse.
Note that the healing comes from the drone, not from Yao.

With her Neuronic buff, the drone will follow your operative around, but it is pretty slow.
For most of the early-game 4* operatives this isn't too much of an issue, but the drone is slow enough that many 5* operatives don't work particularly well with it, as they have too much movement to consistently stay in its AoE.

Ultimate Skill

Yao creates a stationary AoE that heals your operative every 0.5s.
This is Yao healing, not her drone.
If Yao is your deployed operative then she will leave the field, and Yao can't be swapped back in until her ult has finished.

With her Neuronic effect, this also gives your deployed operative CC-resistance.

With only a 10 second duration, the uptime on this skill is extremely low, and as such its average healing is dwarfed by the healing from her Support Skill.
Given that this skill has a U-Energy cost of 100, it's expensive and doesn't add much to Yao's capabilities, meaning it's not a particularly good skill to use.

You'll probably want to save your U-Energy for the ultimate skill of another operative, rather than using it on QQ.'s ultimate.

Weapon

Deep Sea's Call
This 4* weapon gives the deployed operative an ATK buff (based on Yao's ATK) whenever Yao uses her support skill.
The buff lasts 15s, so without any Skill Haste mods the uptime is only 50%, but it's still a nice buff.
With some investment into Skill Haste, the uptime can be higher.
Deep Sea's Call is QQ's best weapon.
Olympus
The game recommends this as her best weapon, but it's not.
Whenever it hits a weakspot, it builds a charge (max 5 charges), and when the operative with Olympus uses their support skill, the charges are consumed to heal you.
Since Yao is a poor dps though, she'll never be on the field to build the charges to begin with.
Similarly, Yao doesn't really need extra healing - her support skill is usually enough.
The healing from Olympus is pretty puny anyway.
As such, Olympus is not a great weapon for Yao.
Only give Yao Olympus if you don't have a Deep Sea's Call yet.

If you don't have either of those, then give Yao whichever sniper you have that gives the highest ATK increase.

For weapon Parts Yao wants max ATK, as Yao and her drones healing scales with her ATK.

Logistics

Squad

Use Moira squad.
It provides your Auxiliary units extra strength and healing effects.
Since Yao's support skill drone is an auxiliary unit, Moira buffs the healing it provides

Stats

Yao is a 4* operative, and as such you will eventually stop using her after you get some 5* operatives.
As such, I recommend you don't waste re-rolls on Yao - they are much better spent on your 5* operatives, if you have any.

Note that the Healing Bonus stat will buff the healing done by Yao, but Yao isn't the one healing - it's the drone from her Support Skill that is healing, so avoid the Healing Bonus stat.

For the Special talent, Yao always wants +10.6% Auxiliary Unit Strength, to buff the healing from her Support Skills drone.

For the Basic Stats Yao always wants +10.0% ATK, as both her own healing and the healing of her drones is based on Yao's ATK stat.

If Yao is using Deep Sea's Call, then for Yao's second Basic Stat, she will want either:
+19.3 Skill Haste
or
+106 Alignment Index

Skill Haste will allow QQ's Support Skill to be used more often, which will give the 15s ATK buff from Deep Sea's Call more uptime.
With three max Skill Haste rolls, the uptime on Deep Sea's Call goes from 50% to 79%.

Alignment Index gives Yao extra healing effect for her support skill drone, which is good as the healing from her drone is the main thing you actually want Yao for, but given how strong of a healer Yao is to begin with, buffing her a little more doesn't make too much of a difference, so getting some more ATK out of her can be more useful.

Whether you prefer healing or ATK is personal preference.

Manifestation

M1 gives an additional heal whenever Yao uses her support skill.
This is a nice burst of healing, but Yao's drone is so strong the extra healing from M1 is often not too important or significant.
M2 automatically deploys a support skill healing drone when your deployed operative gets to below 30% HP.
Quite a nice QoL feature, but for most gameplay this isn't needed.
M3 also grants Yao U-Energy recovery whenever her support skill drone heals
This is useful, but the slow rate at which the drone heals means it's not going to give you U-Energy quickly.
M4 increases the level and healing of Yao's support skill.
M5 increases the healing effect of Yao's support skill drone during its first 5 seconds.
Given its long duration, this isn't that useful.

As a 4* operative, you'll get all of Yao's Manifestations eventually anyway, but none of them fundamentally change how she is played or why you want her on your team.

Good Teammates for QQ

As a healer, QQ works with literally all operatives.

For endgame content QQ isn't as good as Eatchel - The Cub (either as a healer of buffer), and healing isn't often needed anyway, so QQ tends to be more of an early-game healer.

Before you've gotten to max level, gotten Eatchel, or gotten any strong 5* operatives, QQ is a very strong healer, and adding her to all of your teams is perfectly fine, and generally recommended.

Some 5* operatives have to move a lot due to how their skills work, and this can make them more difficult (or at least annoying) to use with QQ.
Examples include Turbo, Dawnwing, Styx Envoy, and Crimson Moon.
Despite this, QQ is still useful and strong even for those operatives, if you need a healer and QQ is all you have.