Vidya - Celestial Swan

If you don't have Vidya - Celestial Swan yet but want to try her out, she is available as a Trial Operative in Story Chapter 15 Pt.1 "Fragments of Truth" (Journey // Philossia)

Celestial Swan is a phenomenally strong support operative that massively debuffs enemies resistances and defence, and who works with all DPS operatives.
If using Celestial Swan as a support then the teams other support operative must either not deal damage or must type-match the DPS, which can make teambuilding difficult for certain operatives. Celestial Swan is currently the games best support operative by a huge margin, so even that is a minor issue for her.

Since Celestial Swan is the games best support operative, you'll find her listed a lot on this site, as she's the best support for literally every dps operative in the game without exception, due to just how strong her debuffing is.

Overall

Role: Support:
DEF and resistance debuffer
Deals Chaos Damage
Weapon Type: Sniper Rifle
Deals basically no damage
(Although mostly skill damage)
Mobility is low

How to use Vidya - Celestial Swan

Use Celestial Swan's ult immediately as the fight starts
Spam her Support Skill off-cooldown
If she has her 5* weapon, her support skill will last indefinitely until the target dies, so you'll only need to use her support skill once.

That's it.

How to build Vidya - Celestial Swan

Weapon: Starlit Truth / Gearlock
Weapon Parts: Max ATK
Logistics Squad: Nebula
+106 Alignment Index
+10.0% ATK
Her Special Stat doesn't matter.
(but ideally +8.5% Chaos Damage)
Best teammates:
Literally everybody

Specifics:

Standard Skill

Tapping the Standard Skill key deploys a holographic landmine in front of Vidya. Holding it causes Vidya to throw the landmine at the targeted location.
(Holographic = They project Celestial Swans model)
Vidya has quite a good throwing arm ( don't ask how she got it ) and can actually throw them quite far.

The landmines detonate when an enemy gets close to them. Vidya can also shoot a mine to detonate it prematurely.

Every 1.5s these are out, these increase in damage and detection/blast radius, maxing after 3.0s deployed.

Vidya can have up to 5 of these landmines up at a time. Deploying more beyond that causes the oldest to be detonated.

Whilst this skill can be quite fun, it can also be clunky and tedious, and its sustained DPS isn't great.
Celestial Swan is incredibly strong when used as a support, but very weak when used as a dps, so you really don't want to ever use her as a dps.

Ultimate and Support Skill

Ultimate Skill

Celestial Swan marks the target as a Prime Suspect.
If that enemies HP gets below a certain amount (determined by Celestial Swan's U-Energy) then Vidya immediately executes them.
(Mechanically, Vidya deals True DMG to the target - meaning it ignores things like DEF and resistances - the target just takes its current HP as damage, thus dying.)
In practice, the HP threshold for this to trigger is so low it's often useless, and it isn't something you want to save your U-Energy for.

Note that Celestial Swans ult has infinite duration - Once marked, an enemy stays marked forever until either Vidya executes them or you use her ult again to mark another target.

Her ult isn't that useful by itself, but it is very useful for proc'ing her M5 Manifestation buff if the target also has Vidya's Support Skill used against them.
With its Neuronic, it also reduces the targets ATK by 10%, which is an ok survivability buff.
It's also useful as it provides an icon above the marked enemy, which can help locate them amidst visual clutter.
As such, you should always immediately use her ultimate skill against the boss a soon as a fight starts against one.

Support Skill

Vidya's Support Skill reduces the targeted enemies DEF, resulting in you dealing a decent amount more damage to them.
In addition, damaging the target causes them to decrease their resistance to the type of damage they took.
This stacks up to 5 times, and the target can gain 1 stack every 2 seconds, however with its Neuronic Vidya can immediately inflict 4 stacks, meaning she can stack the max debuff pretty much instantly.

The DEF debuff is 16%, 28% with M1, or 29.6% with M4.
If Celestial Swan' has M5, then if her support skill is used on the same target as her ult, that target also gains 8% Increased DMG Taken, which is extremely strong.
Each stack of the debuff grants 5% ResPen, for 25% total, or 35% total with M2.
With M3 being at max stacks of the debuff also grant a 15% ATK buff to all other operatives.
With M4 the debuff is increased to 5.5% per stack, for 38.5% total Resistance reduction.

This skill uses S-Energy continuously, and Vidya doesn't recover S-Energy whilst it's active.
It uses 1 S-Energy every 0.2s, for 5/s, meaning it has an uptime of 10 seconds.
This skill has a 5s cooldown after it ends. After the cooldown is over, Vidya recovers 50 S-Energy (which is her max), so she can immediately use it again.
This means this skill has a 10 second duration and a 5 second cooldown, for 66.6% total uptime.
If Vidya has her 5* weapon, then her Support Skill has unlimited duration - it never expires.

Most critically though, is that Celestial Swans Support Skill only provides a damage reduction to one damage type at any given time.
If an enemy is damaged by a new damage type, then it loses its resistance reduction to its previous type.
This means that you can't use multiple different damage types and have Celestial Swan buff all of them.

In practice this means that if you are using Celestial Swan, then you will want your other support operative to either not deal damage, or to deal the same type of damage as your main DPS.
If using Celestial Swan, then having your other support operative and your DPS not type-match is a very significant DPS loss compared to having them type-match, to the extent that if the other support and the dps don't type-match then you're usually better off using another support instead of Celestial Swan, or swapping the other support for a weaker support that does work with Celestial Swan.
This can make teambuilding with Celestial Swan quite complicated, as the main dps will always need to type-match the other support operative.

This also means that some supports work well alongide Celestial Swan simply because they don't deal damage, and thus don't get in the way of Vidya debuffing the damage type of the main DPS. For example Eatchel - The Cub.

Weapon

Starlit Truth
Her 5* weapon, and her best weapon by a huge margin.
Gives Vidya a nice ATK buff and a nice damage buff to the whole squad.
If using her Support Skill while below 20 S-Energy, Vidya gains 1 S-Energy every 0.2s, which is the exact same rate she uses it at.
This means that Starlit Truth gives Celestial Swan infinite duration for her Support Skill.
This is a really nice QoL feature, and not having to spam her skill so often is a very pleasant buff.
When dealing damage during her Support Skill, the weapon also increases damage of that type, and increases the damage enemies take to that type.
This makes it even more critial for Vidya's teammates to type-match each other, beyond the requirement her support skill already has.
Gearlock
Her 4* event weapon.
This gives her 10 S-Energy when her S-Energy falls below 20, once per skill use. This increases the duration of her support skill by 2s, for 70.5% uptime.
It also gives a nice damage buff to the whole squad, which is nice.
Deep Sea's Call
The standard 4* support sniper that gives an ATK buff upon support skill use.
It works well for Vidya, although isn't as good as her designated weapon[s].

For weapon Parts just go with ATK.

Logistics

Use Nebula squad - it was designed specifically for her and is her best choice.
Nebula gives the entire squad more damage, and even more damage if Vidya is below 40 S-Energy, which are very useful.
With her 5* weapon, these are essentially permanent buffs as having her 5* weapon means her Support Skill never expires.

For stats Vidya wants +106 Alignment Index and +10.0% ATK.
For the Special talent, it doesn't matter - all possible stats are literally useless to Vidya when using her as a support operative.
None of these stats change how Vidya plays, and the difference between a Vidya with maxed logistics stats and a Vidya with no useful logistics stats is small.

Alignment Index gives other operatives an extra ATK buff based on Vidya's ATK, at a rate of +1% ATK per 100 Alignment.
This isn't much, but being a passive buff with 100% uptime does make it decent.
A max-roll of +106 Alignment Index will give other operatives an extra 1.06% of Vidya's ATK stat as ATK, which is generally going to be around +60 ATK.
Three max-roll Alignment Index rolls will give your squads other operatives a max of around +200 ATK, which is decent.

ATK is useful for buffing Vidya's passive Alignment buff that gives other operatives ATK based on Vidya's ATK, as well as buffing the threshold for her ultimate.
ATK is worse for Vidya than Alignment, but it's still her second best stat.
Note that Vidya's DEF and Resistance debuffs are not affected by her ATK stat at all.

Since Celestial Swan is a support operative who is never on the field, defensive things like Max HP and DEF are useless to her.
Heal Bonus, Aux Unit Strength, and Shield Construction Efficiency are literally useless to her.
Being a support operative that never deals damage, crit mods are literally useless for Celestial Swan.
(and she deals mostly skill damage when on-field anyway, and skills can't crit)
Since she recovers max S-Energy after her Standard Skill comes off-cooldown, and she can't recover S-Energy whilst it's active, S-Energy mods are useless to her.
Since Celestial Swan doesn't really recover U-Energy, U-Energy Recovery isn't a useful stat for her logistics to have.

Skill Haste is a cope stat if not using her 5* weapon - Celestial Swan has a very high uptime on her support skill anyway, and even higher with her 4* weapon, but Skill Haste will give her more uptime, although it's not a large increase to her uptime.
Since her 5* weapon gives her Support Skill infinite duration, Skill Haste is literally useless if using her 5* weapon.
Similarly, since you'll hopefully get her 5* weapon eventually anyway, getting Skill Haste for her 4* weapon just means she'll need to re-roll later, which is not ideal.
As such, it's better to not go for Skill Haste, just so you don't need to re-roll later, although it's a decent cope stat if you don't have her 5* weapon yet.

For the Special Talent, being an off-field Support Operative who doesn't deal damage makes Celestial Swan incapable of benefitting from any Special Talent in any way.
This means that you can ignore the Special Talent entirely.
You can potentially go for +8.5% Chaos Damage just in case Celestial Swan should ever be on-field for some reason (such as wanting to test the games physics engine...) but if you're using Celestial Swan as your main dps for actual damage then something's already gone very wrong. Just save the Special Talent re-rolls for another operative.

Good DPS Pairings

Celestial Swan is an extremely strong support, and works very well for literally all DPS operatives.
Using Celestial Swan means that you want your other Support Operative to deal the same type of damage as the DPS or to not deal damage at all, but beyond that restriction Celestial Swan doesn't limit your squad in any way.

This type-matching restriction does mean that Celestial Swan does work much better for some operatives than others.
For example Clarina - Crimson Moon + Eatchel - The Cub + Celestial Swan works perfectly, and is Clarina's ideal team.
Alternatively: For Bubu - The Geomancer, since there aren't any strong Ballistic Thermal supports, Bubu is limited to either using strong but off-element supports (like Yao - Nightglow) and not using Celestial Swan, or alternatively using Celestial Swan with a weaker second support like Eatchel - The Cub or Fenny - Starshine.
Celestial Swan can thus make teambuilding somewhat difficult.

Despite her type-matching weakness, Vidya is the games strongest support operative, and is the best support operative for very literally all other operatives without exception.