Acacia - Kaguya

If you don't have Acacia - Kaguya yet but want to try her out, she is available as a Trial Operative in Story Missions 9-3 and 10-12.

Kaguya can debuff enemies damage resistances.
Acacia is best at supporting operatives that deal Frost damage, although she does work with all dps operatives, but she can be built differently depending on whether she is support a frost dps or not.

As a support, Acacia is completely outclassed by Vidya - Celestial Swan, who both provides a stronger resistance debuff, has better uptime on her debuff, reduces enemy DEF, buffs the teams damage, amongst other benefits.
As such, Kaguya is usually only used on teams where Vidya can't be used, or by players who do not have Vidya yet.

Overview

Role: Support - Resistance Debuffer
Deals Frost Damage
Weapon Type: Pistol
Deals both ballistic and skill damage
Rhapsody deals mostly ballistic damage
Her skill damage is also buffed by ballistic buffs
Mobility is:
In combat: Terrible
Otherwise: Very good

How to use Acacia - Kaguya

Hold down the trigger.
If your standard skill isn't active, use it.
If her ult is ready, use it

How to build Acacia - Kaguya

Weapon: Pine Aurora / Prismatic Igniter
Weapon Parts: Max ATK
Logistics Squad: Twilight (Amarna also good)
+19.3 Skill Haste
+14.1% S-Energy Regen
+8.5% Frost Damage
Best teammates:
Any operative that deals Frost damage.
(So Katya or Lyfe - Wild Hunt)

Standard Skill
Acacia throws a projectile that flies around her that deals frost damage and decreases the movement speed of enemies it hits by 10% for 3s, up to 30%.
Each projectile lasts 10s.

With its first Neuronic buff, this skill can have up to 3 charges, and Acacia can have all three projectiles.
With its second Neuronic buff, enemies with three movement speed debuffs are Frozen for 3s, with a 15s ICD.
When shot by Acacia, the Frozen is removed and the enemy deals 200% damage.

Ultimate Skill
Acacia creates an AoE damage in front of her, which deals damage every 0.5s every 8s (or 10s with its Neuronic buff).
With its first Neuronid buff, it also pulls enemies in.

Support Skill
Acacia throws a dart that flies around the enemy, dealing Frost damage and reducing their Resistances to all damage types.

This skill has a 10s duration and a 20s cooldown, so it's uptime isn't overly high by-default.
It has 25s S-Energy cost on a 20s cooldown, so Acacia will eventually run out of S-Energy over extremely long fights. In practice this isn't a problem.

Weapon

Pine Aurora
Her 5* weapon, and her best weapon.
Increases Frost damage - both hers and the full squads.
Increases the full squads damage for 5s after her support skill hits an enemy.
This does refresh duration if applied again, but the long cooldown of Acacia's support skill means this buff doesn't have 100% uptime as she can't maintain it.
Her support skill lasts 10s, then the buff lasts another 5s, on a 20s cooldown. Thus it has a 75% uptime by-default.
Pine Aurora is not an overly strong weapon - it does provide team damage buffs, but they aren't that strong.
Aye, Aye, Captain!
Her 4* event weapon.
Reduces the targets Frost Resistance after damaging them with a support skill.
This debuff lasts 5s.
Her support skill lasts 10s, then the buff lasts another 5s, on a 20s cooldown. Thus it has a 75% uptime by-default.
Acacia's support skill reduces all enemy resistances, but AAC only reduces Frost Res, and hence AAC is only useful if supporting a main dps that also deals Frost damage.

Again: Her support skill lasts 10s, then the above weapon buffs lasts another 5s, on a 20s cooldown.
With some Skill Haste mods, Acacia can maintain these buffs permanently, but not without those Skill Haste mods.

Both Pine Aurora and Aye, Aye, Captain are both quite weak effects.

Prismatic Igniter
Prismatic Igniter gives a 15s ATK buff (based on Acacia's ATK stat) after using her support skill.
Prismatic Igniter is generally better than Aye, Aye, Captain, but if you can maintain AAC's buff with some Skill Haste buff then AAC is still good.
AAC is only useful when supporting a Frost dps, whereas Prismatic Igniter is useful for other damage types too, so Prismatic Igniter is the more versatile weapon.

For weapon Parts just go with max ATK.

Logistics

Twilight
Twilight gives a 5s damage resistance debuff after hitting an enemy with a skill.
Since Acacia's support skill deals damage continuously, this is basically a permanent effect.
Amarna
Gives a 15s ballistic damage buff after using their support skill.
When the support skill deals damage, increases ATK for 1.5s.
Acacia can generally maintain these buffs permanently whilst her support skill is active.
...and Acacia can maintain her support skill indefinitely with some Skill Haste buffs.

Amarna is good when supporting ballistic operatives, but Twilight is good for all operatives.

For the Special talent, Acacia want 8.5% Frost Damage.
For the basic stats, Acacia wants +19.3 Skill Haste.

Skill Haste gives Acacia much higher uptime on her support skill, and Skill Haste is thus her best stat.

Since none of Acacia's debuffs actually scale with her ATK stat, +10.0% ATK is not overly useful for her - it will increase the damage her support skill deals, but even with +ATK it's still not going to be doing a lot of damage.
If using Prismatic Igniter, +ATK is a useful stat, as Prismatic Igniters ATK buff is based on Acacia's ATK.

Alignment Index gives the full squad more damage against enemies inflicted with a Movement Speed Reduction... which her support skill doesn't inflict.
As such, Alignment Index isn't an overly useful stat for her.
Her ult can inflict it, but for shorter fights where you can't use her ult Alignment Index is useless.
Similarly, her ult doesn't really do that much - it just deals damage and reduces their movement speed, which is quite weak for an ult.
Similarly again, since Acacia's ult is quite weak, you are often better off using the ult of your main dps rather than Acacia's.
As such, you may not be able to use Acacia's ult to get any use out of her Alignment Index at all, making any investment into it redundant.

Acacia can make use of S-Energy, as with her Skill Haste mods she can use S-Energy much faster than she uses it.

Skill Haste is her best mod.
S-Energy regen, ATK, and Alignment Index are all ok, but none are nearly as good as Skill Haste.
S-Energy is a good second stat for longer fights
+ATK can allow Acacia to deal a bit more damage, which will be more useful for shorter fights.
Alignment gives a bit more of a damage buff to the main dps after acacia uses her ult
Overall I'd recommend Skill Haste and S-Energy Regen, but ATK and Alignment Index can be better depending on how you intend to use her.

Manifestation

M1 increases the AoE of her support skills dart by 50%.
Not necessarily a dps increase.
With M2 and equipped with a Frost-damage weapon, M2 increases the damage reduction from her support skill by 40%.
This is a nice increase to her damage reduction effect.
M3 increases the duration of her darts by 2s.
A nice duration increase, but it only increases her darts uptime, not the magnitude of their effects.
M4 increases the level of Acacia's support skill, giving it a stronger resistance debuff.
M5 increases the full squads damage to targets inflicted with a movement speed reduction.
This is a 10% damage increase, plus 1% per 100 Alignment Index.

Overall, Acacia's Manifestations are quite weak, and don't do that much to actually make her a better support.

If you want efficiency (good damage without massive investment), then both M2 and M3 are a good stopping points.

Good teammates for Acacia - Kaguya

As mentiuoned in the overview at the top of the page, Acacia is completely outclassed by Vidya - Celestial Swan, who both provides a stronger resistance debuff, has better uptime on her debuff, reduces enemy DEF, buffs the teams damage, amongst other benefits.

This means that the only times where you'd actually want to use Kaguya are

1. If you do not have Vidya yet.
2. If you can't use Vidya - for example Vidya is already being used in your other Neural Sim team that week, and hence can't be used for the second fight.
3. You have significantly and dramatically more investment into Kaguya, and none into Vidya (even then Vidya may still be the better choice...)

Vidya is the better support, but if she can't be used and you have Kaguya, then whilst Kaguya's support skill does reduce all of an enemies resistances, Kaguya herself does deal Frost damage, and both of her signature weapons provide Frost-specific effects.
Simialrly, since Acacia herself deals Frost damage, if using the Twilight logistics squad, Acacia will debuff enemies Frost resistance.
If using Prismatic Igniter and Amarna, then Acacia can be used to support any operative, but with either of her signature weapons or with Twilight squad, Acacia is best used to support a Frost dps.

The list of 5* Frost main-dps operatives is very short though:

Katya - Blue Bolt
Katya - Dawnwing
Katya - Rhapsody
Lyfe - Wild Hunt

Of these, Dawnwing and Rhapsody have much better dps than Blue Bolt and Wild Hunt, but Acacia works with all of them.
If Acacia is using Amarna squad (which provides ballistic buffs), then Blue Bolt and Rhapsody are her best choices, as Blue Bolt deals entirely ballistic damage, Rhapsody deals almost entirely ballistic damage, whereas Wild Hunt deals both ballistic and skill damage, and Dawnwing deals mostly skill damage

Acacia can also be used to support other Frost-damage operatives like Tess - Phantasia, Yao - Quiet Quitter, or Mauxir - Meow, but given that they are all vastly weaker than the above frost dps operatives they're obviously sub-par picks for your dps (and Tess and Yao are better as supports themselves anyway).
Again though - Acacia is a good support for any operative that deals Frost damage.