Bubu - The Geomancer

If you don't have Bubu - The Geomancer yet but want to try her out, she is available as a Trial Operative in all the Story Missions in Chapter 14 Pt.1. I suggest 14A-7.

Bubu deals high damage, has a simple rotation that is easy to play allows you largely ignore the UI and just shoot, reloads automatically when she does basically fucking anything, work well with many strong support operatives, and not needing U-Energy allows your support operatives to use their ultimate's a lot.
Bubu does have lackluster mobility, is quite squishy, and you will have to actually to aim and land every shot, has little variation in how she plays, and has some visual quirks relating to how her paid outfits work.

Providing you are fine with her lackluster mobility and having to manually aim and land every shot, then Bubu is a strong, simple, and competent operative.

How to use Bubu - The Geomancer

1. If either your S-Energy is empty, or your Ultimate is nearly ready and you don't have enough S-Energy to fuel it, then hold your standard skill to fully refill Bubu's S-Energy.
2. If your Ultimate gauge is full and you have enough S-Energy to fuel your entire Ultimate, then use your Ultimate.
3. Otherwise, just shoot stuff.

The Basics

Role: Main DPS
Deals Thermal Damage
Deals entirely Ballistic damage
Mobility:
During ult: Terrible
Otherwise: Standard
Overall: Pretty low

Advantages

High damage
Reloads when she does basically fucking anything
Very easy to play (just shoot stuff)
Very simple rotation / skill usage
You can largely ignore the UI and just shoot stuff
Doesn't need and can't use U-Energy
(Allowing a support operative to use it)
A lot of support operatives work well with her
(Especially the very strong ones)

Disadvantages

You actually have to aim and land every shot
(She's harder to play against mobile/small bosses)
Very low mobility during her Ultimate.
Moving during her ult throws your aim off
(So aiming during her ult can be tricky)
Weak mobility + not overly bulky = kinda squishy
Essentially no variation in how she is played
(Can get boring)
Strafing Left > Strafing Right
Visual quirks related to her paid outfits and ADS

Standard Shot / Passive Effect

Every third shot Bubu fires is an "Arcana Shot" that quickly fires three bullets.
Each of these shots deals less damage than a standard shot, but due to the high fire rate it is still a damage increase over a normal standard shot.

If Bubu doesn't have three bullets in her magazine before firing an Arcana Shot then she will automatically reload before doing the Arcana Shot.
Bubu will also automatically reload after every Arcana Shot as well.
This means that Bubu constantly reloads automatically and you won't need to manually reload - you can literally just hold down the trigger and Bubu will fire indefinitely.
If Bubu is interrupted during a reload, you may need to actually reload manually, but in practice this is extremely rare.

Whenever Bubu fires an Arcana Shot she gains Divine Blessing (which is used for her Ultimate).
(Divine Blessing = The gauge in the middle of the screen.)
Bubu also slowly recovers Diving Blessing over time naturally.

Standard Skill
When Bubu has S-Energy each of her shots use 1 S-Energy to deal more damage and cause her shots to bounce between [up to 4] enemies.
Shots deal less damage after each bounce, but it's still a large damage increase against groups.

Hold Bubu's standard skill to recharge S-Energy (at a rate of 20 S-Energy per second).
Bubu does not recharge S-Energy naturally whilt she's on the field - you need to use her Standard Skill to recharge it.

Make sure Bubu always has S-Energy - do not ever fire shots when Bubu is out of S-Energy.
Similarly, make sure Bubu has enough S-Energy to fuel her entire Ultimate before using her ult, so you don't need to stop in the middle of her ult to recharge her S-Energy.

If a boss has a period of downtime, such as invulnerability, you can use that time to fully recharge Bubu's S-Energy.
You can also fire shots such that the next shot fired is an Arcana Shot, so when the boss' invuln wears off you can immediately shoot an Arcana Shot at them.

Note that Bubu's two headlights glow whilst she has S-Energy, so just glance at her headlights occasionally during a fight and you can largely ignore the games UI.
(Yes, Bubu's literal headlights - the two glowing things on her head.)
Her gun also sounds different when fired with/without S-Energy. This is a good audio clue to tell you that you are out of S-Energy.

Potentially Important Note about Bubu's S-Energy and her Purchased Outfits:
Bubu's standard outfit ("The Geomancer") has headlights, but her other outfits have other areas that glow instead [see images below].
Bubu also has a glowing object float next to her, which disappears when she runs out of S-Energy. These floating objects lag behind Bubu's movement a little.
Outfit Glow Colour Floating Item Glowing Areas
The Geomancer Orange Divination Coin Headlights
Vitality Checkup Pink/Purple Heart The pad on her left thigh
Fizzy Revelry Red/Purple Fruit Her bunny tail
Drizzle Gossamer Orange Divination Coin The left side of her bum-bag
Note the coin in her "Drizzle Gossamer" outfit is the same as in her default outfit - "The Geomancer". I believe this is intentional and not the result of a bug.

If you are ADS-ing, then the floating S-Energy indicators are only visible if Bubu is moving left (i.e. strafing clockwise) - they are not visible when she is not moving left.
(This is true both for her regular shots as well as during her ult.)
As such, I recommend you try to always strafe left if using Bubu, not right (even if you don't tend to ADS, just in case you do ever decide to), as this will keep the object visible.

When Bubu is in ADS and isn't moving left, the only S-Energy indicator that is visible are the headlights on her standard outfit ("The Geomancer") - none of the indicators on her purchased outfits are visible when stationary and ADS-ing. As such, if you normally ADS with Bubu and don't move much, then you may wish to only use her default outfit, as if Bubu isn't strafing left and isn't wearing her default outfit then there is no visual indicator of her S-Energy at all, meaning you'll have to look away from combat at the S-Energy gauge.
You can listen for the sound of her gun, as it changes when she runs out of S-Energy, but if you don't hear that or miss the change then you have no reference at all.
Yes, this does mean that I am saying her free outfit can be easier to use than her purchased outfits.

Is this a massive difference? No.
Is this something that most people won't be affected by in any way? Yes.
You might think it's a small thing, because it is, but for some people which outfit Bubu is wearing and which direction they move will make a difference to how Bubu performs for them.
I wouldn't normally mention something like this in such detail or include so many of the following screenshots, but given that the difference involves outfits that are purchased with actual real money, I do feel it is important to mention, as I really wouldn't want somebody to buy an outfit for Bubu with real money and then be disappointed it didn't work in the same way that her standard outfit did, or felt worse to play (even slightly), and they regretted their purchase, even if slightly.

Ideally I'd like to see an optional part added for each of Bubu's purchased outfits that give her an S-Energy indicator that is also visible when ADS'ing, just like her regular outfit.
(And whilst I am here complaining/dreaming: Have the headlights dim based on her remaining S-Energy to show how much S-Energy she has, not just have them as a binary notification of whether she has any S-Energy remaining at all.)

Bubu's S-Energy indicators:

The Geomancer
Two Headlights and floating coin
(Orange Glow)

Vitality Checkup
Hip-bag and floating heart
(Pink/Purple Glow)

Fizzy Revelry
Tail and floating fruit
(Red/Purple Glow)

Drizzle Gossamer
Bum-bag and floating coin
(Orange glow)

When strafing left Bubu's coin is easily visible.
It also tends to be close to Bubu, so you can quickly and easily glance for it, or notice it in your vision without looking away from the enemy.

When strafing right Bubu's S-Energy indicator coin appears very far to Bubu's left, so you often have to look far away from the enemy to see it.
(It can go off-screen entirely, which is not that uncommon.)

When in ADS, Bubu's coin can be very easy to see when strafing left as it usually appears very close to her.
(This is an extreme example, but it is often this easy to see.)

When in ADS and not strafing left, Bubu's S-Energy indicator coin cannot be seen.
(Yes the left of this image is the left edge of the screen. It's not visible at all.)

Ultimate Skill

Bubu's Ultimate Skill causes her to enter "Evil Purge" state.
Whilst in this state, Bubu's shots are guaranteed to crit, have +50% crit damage amplifier, and use less S-Energy.
These are "Eightfold Arcana Shots" - the name is irrelevant, but know they do count as Arcana Shots and hence are affected by her passive effect and Manifestations.
Each shot uses Diving Blessing - Bubu leaves Evil Purge state when she runs out (or when switched off the field).
Bubu also doesn't recover Diving Blessing by any means when in Evil Purge state, so you can't maintain it indefinitely.

You will note that Bubu gains Divine Blessing whenever she fires an Arcana Shot - regardless of what it hits, if anything.
As such, Bubu can shoot literally anything to recover Divine Blessing, including nothing - merely shooting causes Bubu to gain Diving Blessing.
As such, you can still gain Divine Blessing by shooting invulnerable enemies, missing enemies with your shots entirely, or by shooting the walls, floor, and ceiling - Given that shots from Bubu's ult do significantly outdamage her regular shots, missing her regular shots is not the worst dps loss ever... and that missing even a few shots during her ult can be a large dps loss for her.

Entering Evil Purge state also causes Bubu to reload (because of course she does...)

Bubu is also immobile in this state when firing - If you are holding a movement key then she will dash in that direction when she reloads. This is a small amount of movement.
This fast and sudden movement will throw off her aim, and learning how to keep your aim on-target whilst Bubu vaults around is something you'll have to learn.

Note that a large part of Bubu's damage comes from the shots from her Ultimate due to their guaranteed crit chance and high crit damage.
Bubu's pistol has a default crit damage of 60%, so the +50% amplifier from her ult effectively gives her another +30% Crit DMG, which is pretty nice.

Bubu also creates a large particle cloud when entering her ult, which can make it difficult to see anything.
Further note that Bubu leaves after-images when she moves during her ult, which can make it much more difficult to see what you're shooting at, especially when in ADS.

Bubu creates a large explosion when she ults.
... which can kinda get in the way.
(Especially during ADS.)

Bubu specializes in divination, but her hidden mastery is in eye-cancer.
( What is Bubu even shooting at here..? )

Support Skill
Bubu's Support Skill creates a 10s AoE that deals Thermal DMG to enemies in it, as well as dealing Thermal DMG again when enemies in it are shot.
The first is 20% of Bubu's ATK every 0.5s, for 400% of Bubu's ATK, and the per-shot DMG is 100% of Bubu's ATK +347.

Overall, Bubu's support skill is quite strong as far as as the non-buffing/debuffing support skills go (not that that says anything...), but it's still not something you'd actually want to use a support operative slot to get.

Weapon

Gilded Blaze
Her 5* weapon, and her best weapon.
Plum Blossom
Her 4* event weapon.
Note this inflicts Stun every 60 shots. Inflicting Stun on an enemy recharges 1 Divine Seal (1/3rd of her Ultimate) which is good.
Not as good as her 5*, but still good.

For weapon Parts just go with max ATK.

Logistics

Use Halo squad - it was designed specifically for her and is her best choice.

For stats you want 10.0% ATK and +10.6% Crit Damage.
For the Special talent, you want 8.5% Thermal Damage

Since such a large percentage of Bubu's damage is in her ult, which always crits, crit rate only actually affects Bubu's standard shots, resulting in a small overall bonus.
Crit Rate is thus a weak cope stat for Bubu.

106 Alignment gives her 5.3% ATK, however enough of Bubu's damage is in her Ultimate (which always crits) that Crit Damage is the better option for her.
Alignment Index is still a good stat - it's just not quite as good as Crit Damage.
Since her pistol has 60% crit damage, 10.6% Crit Damage Amplifier is +6.36% Crit Damage.
Note that even with the +50% from her ult, Bubu still has more ATK buffs than Crit DMG Amplifier buffs, meaning ATK is a more saturated buff category, and hence Crit DMG Amplifier is worth a bit more due to the two stacking multiplicatively with each other.
Bubu also gains extra crit damage from her M1 as well, making Crit Damage Amplifier even stronger.

Manifestation

M1 gives Bubu +20% Crit DMG, plus an extra 20% Crit DMG to her Arcana Shots.
Given that her Arcana shots are where almost all of Bubu's damage comes from, 20% crit dmg is very nice, especially for her ult as those always crit.
Her ult also gains extra crit damage amplifier, making the extra crit damage very useful.
M2 gives Bubu +12% Final damage for her Arcana shots.
Again, this affects the vast majority of her shots, and hence is a good bonus - maybe a 10% Final Damage Bonus or so.
This also gives Bubu CC-resistance when in her ult, which is nice.
M3 gives Bubu 20% ResPen when equipped with a Thermal Weapon (so... always).
A very nice buff.
M4 gives Bubu +1 to her standard skill level, and the damage bonus goes from 47.5% to 57.5% (a 21.05% bonus)
Since Bubu should always have S-Energy when shooting if played properly, this should affect all of her shots.
M5 gives Bubu +20% ATK for 10s after an Arcana Shot crits.
Given that Bubu's ult always crits, this is essentially a permanent buff.

Overall, Bubu's Manifestations are on the weaker side when compared to many other newer operatives, but they're still a large overall bonus to her capabilities.

If you don't want to get her all the way to M5, then I'd say M3 is a good stopping point.

Good Supports for Bubu - The Geomancer

Bubu works well with many of the games strong support operatives:

Vidya - Celestial Swan (and other resistance shredders)
Celestial Swan and other resistance shredders (Acacia - Kaguya, etc.) that reduce the enemies resistance help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off element to Bubu.
Yao - Nightglow
Bubu can make full use of Nightglow's ATK, Ballistic, and crit buffs, and Nightglow is a fantastic support for Bubu.
Note that Nightglow is off-element to Bubu (Nightglow deals Electrical damage)
Eatchel - The Cub
A Blitzing Fangs Eatchel provides healing as well as a decent ATK buff.
Not the strongest support, but still good.
Mauxir - Shadow Ka
Bubu really likes the utility of not having to aim, and the extra damage is nice.
Note that Shadow Ka is off-element to Bubu (Shadow Ka deals Chaos damage)

Bubu's best supports are Celestial Swan and Nightglow, although since Nightglow is off-element to Bubu you won't want to use both Celestial Swan and Nightglow together.
Mauxir isn't as mechanically strong, but if she helps you land more shots against tricky/mobile bosses then she will likely be one of your best supports.
Celestial Swan + Eatchel is a great combo, as is any combination of Kaguya, Nightglow, Eatchel, and Shadow Ka.

Cope Supports like Agave, [Redacted], Quiet Quitter, etc. are still fine if they're all you have.

Since Bubu reloads automatically and has a tiny magazine size, Fenny - Starshine is a relatively poor support for Bubu.
Starshines ballistic buffs are still good, but since Starshine's buffs only apply when an operative has one of her special Clips and Bubu will empty that special Clip in 6 or 7 shots, Starshine requires a lot of clicking to constantly re-apply her special Clip, and the very high rate that you will need to spam her support skill at means Starshine will run out of S-Energy extremely quickly, which massively reduces how useful she actually is.
Similarly, since Bubu reloads automatically, Starshine doesn't remove any reload time from Bubus rotation like she can do with other ballistic operatives.

Tess - Phantasia works with Bubu by buffing all her shots, but being off-element means Bubu + Phantasia doesn't work with Celestial Swan.
Similarly, Bubu's relatively low RoF means Phantasia isn't as good for Bubu as she is for operative with fully-automatic weapons, and Phantasia is a very weak support for Bubu.
Fritia - Little Sunshine also technically works with Bubu - she's not as strong as Tess, but is on-element and is thus one of the better 4* supports for Bubu, not that that says much...