Nerida - Styx Envoy

If you don't have her yet but want to try her out, Nerida is available as a Trial Operative in Story Chapter SP5 "Abyssal Dawn" ( Main Story // Journey / Philossia // Abyssal Dawn )

Nerida deals high damage, has very long range, and has incredible mobility, but the way Nerida works means that she really really wants Marian - Riptide as her Sub-DPS/Support, and for a large part of her rotation she and Marian basically have to remain stationary for a while, which does reduce her survivability, which is otherwise excellent due to her fantastic mobility.
Nerida really wanting Riptide as her support means that Nerida's ideal teambuilding options are terrible, but with her ideal supports she works very well.

Nerida is generally the only Main DPS operative who actually has a viable use case for her Support Skill - for basically every other Main DPS operative, their Support Skill is something you will never want to use under any circumstances.

The Basics

Role: Main DPS
Deals Chaos Damage
Weapon Type: Assault Rifle
Deals entirely skill damage
Mobility:
During ult: Nerida: Poor
Marian - Riptide: Decent
Otherwise: Incredible

How to build Nerida - Styx Envoy

Weapon: Shadow Surge / Web Snare
Weapon Parts: Max ATK
Logistics Squad: Sidhe
Basic: +10.0% ATK
+106 Alignment Index
Special: +8.5% Chaos Damage
Best supports:
Marian - Riptide
Vidya - Celestial Swan

How to use Nerida - Styx Envoy

Use Nerida's Standard Skill until both:
Nerida's Wail Charge gauge is full
and
Marian - Riptide is at full Tidesong.
(Around 3.5 Tidesong is fine - see below.)
Use Nerida's ult
Immediately hold Nerida's standard skill to detonate all her ribbons
Swap in Marian - Riptide via her Support Skill and magdump as fast as she can
Immediately swap Nerida back in via her Support Skill.

Repeat

Staying close to the enemy can help with Nerida's Support Skill hitting the enemy before her ult finishes, increasing damage a little bit since Nerida's Support Skill is Chaos Skill Damage, and is thus buffed by her ult.
This isn't necessary, and isn't a large dps increase, but it does help her dps a little.
Marian - Riptide's Manifestations (notably M1) will increase the speed at which she can magdump a full Kraken mag, so without investment in Riptide's Manifestations you might not be able to hit Nerida's Support Skill during her ult, regardless of how close you are.

Since Nerida's Ult is itself a skill cast, you only need Marian - Riptide to be at around 3.5 Tidesong when Nerida uses her ult in order for Riptide to be at max Tidesong by the time Riptide is actually swapped in.
Riptide gains Tidesong faster with M3, which will affect how much Tidesong Riptide needs.
You'll get used to this quickly, and it'll become intuitive to use Nerida at the correct Tidesong without thinking.

Specifics:

Passive Effect

Whenever Nerida deals skill damage while equipped with a Chaos weapon (i.e. hopefully always) her damage also inflicts an additional 80% of the damage inflicted as "Wrath Haunt", which deals DoT damage.

If an enemies HP is less than the Wraith Haunt, then the Wraith Haunt is very quickly converted to damage, basically acting as a pseudo-instakill finisher for weak enemies.

Standard Skill

Tapping her standard skill causes Nerida to create a Phantom every 0.5s that fires ribbons at several targets in front of her, dealing Chaos damage.
Each phantom she creates counts as a standard skill use, so Nerida uses a standard skill every 0.5s.
This gives her "Wail Charge" and a little U-Energy.
Wail Charge is the gauge in the centre of the screen.

Holding her standard skill causes Nerida to consume all her Wail Charge to deal damage to enemies in front of her based on how much Wail Charge she used.
This also recovers all her S-Energy.

Whilst using her standard skill, Nerida can't be interrupted, but also can't regen S-Energy.
She also quickly moves around in the targeted direction, giving Nerida incredible mobility.

Ultimate Skill

Nerida creates a "Truth-Seer plasma jellyfish" (I.e. big AoE thingy) which causes Wraith Haunt to be increased from 0.8x to 1.8x but not cause DoT damage, as well as causing Wraith Haunt to be inflicted by any operatives Chaos Skill DMG, not just Nerida's.
With its second Neuronic, Nerida using her ult also causes all previously inflicted damage to also be increased to the 1.8x multiplier.
When Truth-Seer ends, all the Wraith Haunt is immediately converted to damage.

With its first Neuronic, any target in Nerida's ult that has Wraith Haunt greater than its current HP is immediately defeated.
This can sometimes bypass invuln, but only rarely.

Many enemies become invulnerable once their HP bar reaches a certain threshold, or when their shield is broken, but this often doesn't remove the Wraith Haunt already inflicted upon them.
This existing Wraith Haunt can thus instakill them the moment they become vulnerable again.
This can make it much easier for Nerida to defeat many bosses, as she often doesn't have to do anything to defeat their second phase - her Wraith Haunt will simply defeat the enemy automatically as soon as it becomes vulnerable.

Nerida's ult lasts very slightly longer than it takes for Nerida to hold her standard skill, swap Marian in and magdump, and then swap Nerida back in via her support skill.

Support Skill

Nerida fires ribbons in front of her, dealing chaos damage.
This also swaps Nerida in.

Nerida's Support Skill allows a very quick and easy way of swapping her back onto the field, which is useful for getting Nerida back on-field after using her ult and then having Riptide swap in to magdump her Kraken mag.
That this skill deals Chaos Skill Damage means that if you do her rotation fast enough, Nerida swapping back in via her Support Skill can deal damage whilst her ult is still active.

Weapons

Shadow Surge
Her signature 5*, and her best weapon.
Shadow Surge increases Nerida's Chaos DMG and ATK.
Shadow Surge causes enemies to take increased skill damage, which is the strongest effect the weapon gives.
Web Snare
Her signature 4*, which gives her a nice Chaos DMG and ATK boost.

If you don't have either of those, then you can just use Ashen Dog for the Chaos damage boost - none of the 4* AR's really give Nerida much benefit.

For Weapon Parts Nerida wants maximum ATK.

Logistics

Use Sidhe squad - it was designed specifically for her and is her best choice.
Sidhe gives Nerida a very large 65% Skill DMG boost (doubled during her ult) which is a very strong damage increase, and gives Nerida far more damage than than any other squad.

For stats Nerida wants +10.0% ATK and +106 Alignment Index.
For the Special talent, Nerida wants +8.5% Chaos Damage

Alignment Index gives Nerida more ATK, which is very useful.
A max-roll of +106 Alignment Index gives Nerida +5.3% ATK.

Nerida recovers her S-Energy at the end of her rotation, and hence S-Energy regen is useless to her.
Nerida's standard skill generates enough U-Energy for her ult, so U-Energy regen is useless to her.
Since skill dmg can't crit, and Nerida only deals skill dmg, skill dmg mods are useless to her.

Nerida's rotation is limited by the rate at which she can generate Wail Charge, and the rate at which Riptide can generate Tidesong, so reducing her skill cooldowns with Skill Haste doesn't help Nerida's damage.

Manifestation

M1 increases Nerida's damage by 25%
M2 causes Nerida's standard skills to bypass 15% of the targets chaos res.
This isn't quite a 15% damage boost (especially as it doesn't buff Riptide), but it's still nice.
M3 causes Nerida's Skill damage to ignore 20% of a targets DEF when equipped with a Chaos Weapon (I.e. hopefully always)
This does buff all of Nerida's damage, but doesn't buff Riptide.
M4 increases the skill level and damage of Nerida's standard skill by the usual 10%
M5 increases the entire squad's ATK by 8% of Nerida's ATK during her ult.
This is good for Nerida, but also good for Marian - Riptide as well.

If you want efficiency (good damage with minimal investment), then M3 is a good stopping point.

Good Supports for Nerida - Styx Envoy

Marian - Riptide
In case it wasn't obvious by now, Marian - Riptide is Nerida's best partner by-far.
Styx Envoy was basically designed around having Riptide as her Sub-DPS, and Nerida performs vastly worse if Riptide isn't by her side.

If you don't have or otherwise can't use Marian - Riptide alongside Nerida, then I recommend that you do not use Nerida.

Alongside Riptide, most of the usual skill dmg supports work well:
Vidya - Celestial Swan
Vidya massive reduces the enemies DEF and Resistance, and buffs the teams damage a little too.
Vidya only debuffs the damage resistance of the damage type the enemy has most recently received.
As such, if using Celestial Swan, then make sure the other non-Vidya support isn't off-element to Nerida.
(i.e. They either deal Thermal damage or do not deal damage at all).
Since both Nerida and Riptide both deal Chaos Damage, there's no worry about losing Celestial Swans buffs by being off-element.

Other good supports include:

Siris - Freefin
Freefin provides strong skill damage buffs, although Nerida can't make use of Freefin's Wavehunt thermal skill damage buff.
Marian - Aquila
Aquila provides very strong generic damage buffs, which makes her an excellent support.
Aquila is off-element to Nerida (Aquila constantly deals Kinetic damage).
Eatchel - The Cub
A nice passive ATK buff.
Acacia - Kaguya
Kaguya is a good resistance debuffer - not nearly as strong as Vidya - Celestial Swan, but if you aren't using Vidya then Kaguya can be used as a much weaker substitute.
Mauxir - Shadow Ka
More damage, and more Chaos Skill DMG is very nice.
Chenxing - The Observer
Skill DMG and Standard Skill DMG buffs are great for Nerida, although keeping Nerida inside Chenxing's PBAoE can be very difficult, so she's not a good choice.