Cherno - Those Two

If you don't have Those Two yet but want to try her out, she is available as a Trial Operative in Story Chapter 5, Missions 1 to 8.

Those Two deals decent damage with her standard skill albeit on a long cooldown that gives very low uptime, and her ult deals good damage whilst also healing her albeit with a high U-Energy cost, although she performs quite poorly without investment into her Manifestations, and by the time you get her Manifestations you'll likely have 5* operative who will massively outperform her

The Basics

Role: Main DPS
or a generic support
Deals Chaos Damage
Weapon Type: SMG
Deals both ballistic and skill damage
Mobility is good

How to use Cherno - Those Two

Just spam all her skills off-cooldown
Otherwise, just Shoot stuff

How to build Cherno - Those Two

Weapon:
As a DPS: Alloy Truth / Hypochlorous Acid.
As a support Alloy Truth / Frigatebird.
Weapon Parts:
Alloy Truth: Max HP
Otherwise: Max ATK
Logistics Squad: Navigator
Basic Stats: +19.3 Skill Haste
+10.0% Max HP if using Alloy Truth
+10.0% ATK otherwise
Special Stat: +8.5% Chaos Damage
Best squadmates:
As a main-DPS: Chenxing - The Obeserver
Marian - Queen of Pain
Yao - Quiet Quitter
As a support: Basically all dps operatives.

Standard Skill

Cherno deals DoT Chaos damage to an area in front of her.
This deals damage every 0.3s, so it will deal damage 4 times over its 1.25s duration.
When the effect ends, it deals damage again, whilst also interrupting foes.

This skill deals quite high damage for a 4*, and its 15s cooldown is quite short, but since it only lasts 1.25s, it has extremely low uptime.

Ultimate Skill

Cherno summons Beloberg to attack enemies, with each attack using a little of Cherno's HP.
Beloberg lasts 15s, or until Cherno drops below 25% HP.
Cherno's ultimate has a 10s cooldown, but that cooldown starts when Beloberg disappears, not when the skill is first used, so in practice it's cooldown is much longer than 10s.

This ult is strong for a 4* operative, although its high cost of 80 U-Energy means you can only cast it infrequently, so its dps isn't that great.
Note that the damage of Beloberg's attacks is based on Cherno's Max HP, not her ATK stat. This makes Cherno's HP a stat you really want to increase, although since Cherno's attacks still rely on her ATK, you do still want to buff her ATK if using her as a main dps.

Support Skill

Cherno becomes incorporeal and dashes through enemies, slowing them for 3s each.
With her Neuronic, Cherno also recovers 5% of her max HP for each hit, which can result in a lot of healing for her if used against a group, although since Cherno needs to be off-field to use her support skill and it only heals herself, it's not a great healing ability as off-field support operatives generally don't need healing.

Weapon

As a DPS:

Alloy Truth
Gives a nice ATK buff to the entire squad, based on Cherno's Max HP, as well as a nice Crit DMG Amplifier buff.
100 Battle Veteran
100BV deals more damage the more empty the magazine is, and is a very strong SMG for cherno.
Hypochlorous Acid
Gives Cherno a Max HP buff based upon her ATK.
Given that her ults damage is based on her HP stat, this is useful.

As a Support:

Alloy Truth
Gives a nice ATK buff to the entire squad, based on Cherno's Max HP, as well as a nice Crit DMG Amplifier buff.
Frigatebird
Gives the deployed operative a 15s ATK buff when Cherno uses her support skill.
This is a nice buff, but it scale off of Cherno's ATK, whereas if using her as a main dps Cherno wants max HP, so it's not as ideal as Alloy Truth which works perfectly for Cherno as both a dps and a support.

For Weapon Parts, if using Alloy Truth then you want Max HP, otherwise max ATK.

Logistics

Navigator squad is her best logistics squad.
With Navigator squad, inflicting a control effect on an enemy increases the damage they take for 10s.

Given that both her support skill and ultimate skill inflict Slow, Cherno can proc this pretty easily.

For her Special Talent, Cherno wants +8.5% Chaos Damage.

Cherno wants Skill Haste to be able to spam her Support Skill as often as she can, thus keeping the debuff from her Navigator squad active as much as possible.
Since it has a 25s base cooldown, you can't get it down to 10s cooldown, but you can reduce its cooldown significantly with Skill Haste mods.

You will also want a stat to buff the buff from her weapon.
If using Frigatebird then this means she wants ATK, and if using Alloy Truth she wants Max HP.

It's generally not worth re-rolling stats for 4* operatives though, as once you get 5* operatives you'll stop using 4* operatives due to just how much better the 5* operatives are.
As such, you should really just give Cherno Max HP for when she eventually gets Alloy Truth.

Manifestation

M1 removes the time limit on her ult, meaning it will only disappear once Cherno reaches 25% HP.
With a decent healer, the uptime can be very good on her ult, giving her high dps from her ult.
With a very strong healer, Cherno can maintain this indefinitely, giving her much higher dps.
M2 fully heals Cherno when she uses her ult.
This makes it quite easy to get very high uptime on her ult, but if you are using her with a healer then M2 doesn't do much.
M3 gives Beloberg +1% damage per second, up to 25%.
This only works if Cherno has a Chaos-damage weapon, but if you've gotten her to M3 that shouldn't be too difficult.
M4 gives Cherno's ult +1 level and increasing its damage.
M5 gives Cherno extra ATK equal to 1% of her Max HP.
This is a good bonus.

Being a 4* operative, you'll max all her Manifestations eventually.

Cherno's Manifestations just buff her ult, so you'll ideally want to pair Cherno with a good healer so she can use her M1 buff to maintain her ult as long as possible, although that isn't require.

Good Teammates for Cherno - Those Two

If Cherno has her first Manifestation, then giving her a strong healer can allow her to sustain her ult for much longer, massively increasing her dps.
As such, Eatchel - The Cub and Yao- Quiet Quitter are great supports for Cherno, and Chenxing - The Observer is also quite good.

As a main dps Cherno spends a lot of time in combat shooting stuff, so ballistic and per-shot buffs work well, such as Tess - Phantasia and Fritia - Little Sunshine.

Since a lot of damage comes from her standard and ultimate skill, skill damage buffs such as Siris - Freefin, Marian - Riptide, and Chenxing - The Observer work well with her.

Operatives that can reduce enemies chaos damage resistance such as Vidya - Celestial Swan, Acacia - Kaguya, or Marian - Queen of Pain also work well with Cherno.

If you only have 4* operatives to support Cherno, then her best support is Fritia - Little Sunshine, who adds Thermal damage to each shot.
From there you will likely want a healer, so take Yao - Quiet Quitter.

As a support, Cherno is a very generic support, providing Slow, damage taken debuffs, and ATK buffs, which can be used by basically all dps operatives.
As such, she can be used to support basically any operative, although she isn't a particularly strong support.