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 side 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.

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 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.
Use Nerida's ult
Immediately hold Nerida's standard skill to detonate all her ribbons
Swap Riptide in 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.

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
Siris - Freefin
Marian - Aquila
Eatchel - The Cub

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.

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 a Neuronic, Nerida using her ult also causes all previously inflicted damage to be increased to the 1.8x multiplier.
When Truth-Seer ends, all the Wraith Haunt is immediately converted to damage.

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

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 in via her support skill.

Support Skill

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

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 max 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 makes it amongst the largest dps-increases from a logistics squad in the game.

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.

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
DEF and Resistance shredding are excellent for Nerida
Since both Nerida and Riptide both deal Chaos Damage, there's no worry about losing Celestial Swans buffs by being off-element.
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 is good for resistance debuffing if Celestial Swan is busy that week.
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.