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

Whilst QQ is basially useless as a DPS, she functions as a fantastic healer.
At endgame healers aren't needed, and Eatchel - The Cub is both a better healer than QQ but also provides useful buffs, but if you do actually need two healers then QQ is always your second choice, and before you get Eatchel QQ will be your best healer by a huge margin.
If given a specific 4* weapon, she can also provide a moderate ATK buff to your dps as well.

Overall

Role: Healer
Frost Damage
Deals f**k all damage
Mobility:
Herself: Average
Her Drone: Slightly above average

Advantages

Second best healer in the game
Before you get Eatchel she's the best healer in the game by a huge margin

Disadvantages

Healing isn't overly useful at endgame
If you do want a healer at endgame you'd use Eatchel
Offers very little besides healing
PBAoE healing can be annoying for some 5*s

How to use Yao - Quiet Quitter

1. Add her to your team
2. If you need healing, use her support skill and stand in its AoE
3. If Yao has a support weapon or logistics set, then spam her support skill off-cooldown

Standard Skill
Yao deploys a "Slackers Aid" for 10s, which
1. Continuously replenishes her ammunition
2. Adds Frost damage to 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.
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 can pretty easily be over 1,000 healing per second.
Note that the healing comes from the drone, not from Yao.

With her Neuronic buff, the drone will follow your operative, 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, she will leave the field and can't be swapped back in until her ult has finished.

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

With only a 10 second duration, the uptime on this skill is extremely low, and as such its 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 Olympus hits a weakspot, if 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.

For weapon Parts go with max ATK.

Logistics

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

For stats
If you are using Deep Sea's Call, then you want ATK + Skill Haste + Aux Unit Strength
If you aren't using Deep Sea's Call, then you want 10.0% ATK and +106 Alignment Index.
Although since you'll eventually get Deep Sea's Call, you're better off going with Skill Haste to begin with rather than going with Alignment then having to change later.
For the Special talent, you want 10.6% Auxiliary Unit Strength

Yao's healing is based off her her ATK, so more ATK = more healing
Auxiliary Unit Strength is good as that buffs the healing done by her support skill drone.
Note that the Healing Bonus stat will buff the healing done by Yao, but Yao isn't the one healing - it's her drone that is healing, so avoid the Healing Bonus stat.

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.

If you are using Deep Sea's Call, then you should use Skill Haste instead of Alignment Index.
Skill Haste will allow QQ's Support Skill to be used more often, which will give the 15s buff from Deep Sea's Call more uptime.

Losing the Alignment index does mean that QQ will heal less, but it's a small loss of healing.
With three max Skill Haste rolls, the uptime on Deep Sea's Call goes from 50% to 79%.

Overall, just go with ATK and Skill Haste - again, it's not worth investing into Alignment Index, as you'll need to change later anyway.

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
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.

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 all operatives.

For endgame content QQ isn't as good as Eatchel (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 often 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.