Clarina - Crimson Moon

If you don't have Clarina - Crimson Moon yet but want to try her out, she is available as a Trial Operative in Story Mission SP6-8

Overall, Clarina is quite a good operative - she has good damage, incredible mobility, high defence, heals herself a lot, and her constant damage makes her very fun to play - She's a good choice for casual play and can be very capable for endgame content.
Constantly damaging herself means that her constant healing doesn't translate into the massive survivability bonus you might expect for such high healing amounts though, her mobility is only good if you aren't using her ult, which requires her to stand in place and just hope the enemies don't punch her in the face too hard before it's over, her constant movement can get annoying to play, as can the constant clicking needed to play her, and teambuilding with her is quite difficult due to just how few types of buff actually affect her.
Overall, Clarina is a good operative for both casual and endgame play, but she has a few annoyances and caveats.

How to use Clarina - Crimson Moon

The Basics

Role: Main DPS
Deals Kinetic Damage
Weapon Type: SMG
Deals Skill Damage only
Mobility
During Ult: Literally zero
Otherwise: Incredible
Can self-heal

How to use Clarina - Crimson Moon

If her Crimson (ult) gauge is full, use her Ult
Otherwise, spam her Standard Skill and click repeatedly

Repeat

Notes:

Clarina's gun is utterly useless, and using it will cause her skill buffs to time out, so shooting will massively reducing her damage.

Clarina's rotation takes around 15 seconds
(1.5s per Standard Skill (for 9s total), then around 6s for her ultimate)

During each 15s rotation, Clarina will:
take 5*20%=100% of her current HP as self-damage
heal 3,960*5 + 10,990 HP = 30,790 HP
(or 31,900 if she has M4).

How to build Clarina - Crimson Moon

Weapon: Rosewing / Blight Briar
Weapon Parts: Max ATK
Logistics Squad: Hymn
Basic Stats: +10.0% ATK
+106 Alignment Index
Special Stat: +8.5% Kinetic Damage
Best supports:
Vidya - Celestial Swan
Eatchel - The Cub

Passive Effect

Clarina has a Crimson Charge gauge.
Using her Standard Skill or her Dodge-shot gives her Crimson Charge.
Crimson Charge is used for her Ultimate.

The max Crimson Charge is 120.

If Clarina has a Kinetic weapon (i.e. hopefully always), then each time Clarina is healed, for each 10 HP Clarina gets when healed she gets +1 Max HP, up to 3,000 max.
Clarina loses 30% of this bonux Max HP each second.
For each 100 bonus Max HP Clarina has, she gets Skill DMG +8%.
(Yes, the skill says "Final Active Skill DMG, but note that all of Clarina's skills are Active...)
At the max of +3,000 Max HP, this would give Clarina +240% Skill DMG.
Maxing Clarina's Deiwos requires 9,000 healing per second, which is not feasible during normal gameplay.
For more on this, see the later section "Deiwos and Passive Effect", as this passive is actually one of Clarina's most important features.

Standard Skill / Dodge

Clarina's Standard Skill and Dodge causes her to "twirl forwards" (yes that's an actual quote from the skill description, and I love it) - essentially a quick dash.
Her Standard Skill isn't a dodge - she can't be interrupted during it, but can still take damage. Her dodge still functions as a dodge.
This twirling dodge does replace her regular dodge, meaning that Clarina can't just easily/quickly dodge-cancel out of the way of certain things like other operatives can.
With Clarina, if you dodge then you either dodge and commit to the 15 rounds burst fire or you dodge and wait until she has stopped moving before you can act.
Both of these take more time than other operatives standard dodges.
During both her dodge and her Standard Skill, Clarina summons 3 Bats that fly towards the target enemy. Each Bat that hits grants her 3 Crimson Charge (so 9 total)

When Clarina uses her Standard Skill or her Dodge, she has a timer appear on-screen.
Shooting during the center section of this timer will cause Clarina to twirl in the target direction and quickly unload 15 bullets at the target which doesn't deplete her magazine.
Shooting when the timer isn't in the center section won't activate the effect, but it won't cancel the timer either, so you can just spam click to activate it as soon as possible.
Since most of Clarina's rotation is spamming her standard skill, this translates to a lot of clicking, which can make Clarina annoying to play over long periods.

Dodge: The effect will recover 1 U-Energy, 5 S-Energy, and 6 Crimson Charge, and will also heal Clarina for a fixed amount.
This healing is 27* Clarina's Level + 216 = 2,376 at max level
Each shot does also deal AoE Kinetic DMG around the target.
This does count as Standard Skill DMG, and does also count as a Standard Skill use, but doesn't count as a shot or hit.
This means "on-hit" effects like Tess - Phantasia don't affect Clarina, which does limit her pool of viable support operatives.
Standard Skill: Each bullet will recover 0.2 U-Energy, 1 S-Energy, and 1 Crimson Charge, and will also heal Clarina for a fixed amount.
This healing is 3x Clarina's Level +24. This is unaffected by any +Healing modifiers, but does count as healing.
At Max level this is 264 HP per round. Over 15 rounds this is 15*264 = 3,960 HP.
This does count as Standard Skill DMG as you'd expect, but again it doesn't count as a Standard Skill hit or shot hit.
This means "on-hit" effects like Tess - Phantasia don't affect Clarina, which does limit her pool of viable support operatives.
Note that Clarina's dodge-shot only deals around 50% of the damage of her Standard Skill, and only generates a max of 15 Crimson Charge, meaning Clarina should ideally never use her dodge-shot, as it deals less damage and reduces the frequency of her ult (which is where most of her damage is).

Auto-Aim

Note that both of these effects (her Standard Skill and her dodge-shot) have auto-aim, which is actually more of a pseudo-auto-lock-on - you can still miss with her skill, but given the auto-lock-on and the skills extreme rate of fire it's very unlikely, and chances are that you'll hit all her bullets basically every time. Still not guaranteed though.
Most notably, Clarina's bullets can still be blocked by terrain.

HP Loss, Gain, and Average HP

Note that using her Standard Skill and dodge causes Clarina to lose 20% of her current HP.
When below 20% of her max HP, she doesn't lose any HP, but her standard skill and dodge deal less damage.
Clarina thus loses more HP the more HP she has, whereas her healing is fixed.
This means Clarina tends to gain a lot of HP when low on HP, and tends to on-average lose a little HP when her HP is full.

Mathematically, with no external healing, Clarina's Standard Skill healing of 3,960 means that Clarina's HP will asymptote to 19,800 whilst she is spamming her Standard Skill.
If she has more HP than that, she will lose HP.
If she has less HP than that, she will gain HP.
If you just spam her Standard Skill, her HP will oscillate around that number.

At Lvl.80, Clarina has a max base HP of 22,911, which after factoring in her Logistics bonus puts her at 27,493 HP, which is very low, especially for a main DPS.
With maxed Manifestations, Clarina has a max HP of 28,867, which is again, quite low for a main DPS.
Clarina does gain Max HP from her Deiwos, but overall her HP is still quite low.
Despite this, at Lvl.80 Clarina has a base DEF of 1,272, which is very high. With the 20% Logistics bonus, Clarina has 1,526 DEF, which is extremely high.
As such, Clarina tends to have low-ish HP but high DEF.
Her high DEF combined with her self-healing does make her much more bulky than a typical operative.

Having to rely on her high DEF and the average of her self-healing does make Clarina very vulnerable to effects that reduce DEF and/or self-healing.
Having either such effect can be a huge nerf to Clarina's survivability.

Note that some effects that reduce how much healing Clarina gains can cause her HP to drop quite quickly by using her standard skill, leading to extremely low survivability.

Ultimate Skill

Clarina deploys her parasol to fire shots at the target.
Clarina's ult does have a little bit of auto-aim, so her aim is easier to control during her ult than Bubu, but it's still entirely possible to miss or have her shot be blocked by terrain.

Each shot of her ult uses 24 Crimson Charge.
Given that Clarina has a max of 120 Crimson Charge, her ult can fire a maximum of 5 shots.

The first 3 shots from her ult all do the same damage, with the 4th shot dealing a lot more, and her 5th dealing even more still.
Each of the first 3 shots are 12% of her ults damage, the 4th shot is 25%, and the 5th shot is 39%.
You want to only ever use Clarina's ult when she has max Crimson Charge, in order to maximize her ults damage.

Most of Clarina's damage comes from her ult.

Also, yes, her max Crimson Charge is 120, and she gains 3*3=9 from using her skill and another 15 for each shot that hits, meaning that her standard skill gives her 24 Crimson Charge, which is exactly enough for 1 shot of her ult.
As such, you can use her standard skill 5 times and you'll have enough to have her Crimson Charge maxed so she can ult with max charge.
This is Clarina's standard damage rotation: 5 standard skill uses and then use her ult, repeat.
This does also mean that missing even 1 single bullet from any of her standard skill uses does mean you'll have to use her standard skill a 6th time to get to max.
Since most of Clarina's damage is in her ult, having to delay her ult to do a 6th standard skill use does reduce her dps by quite a lot.

Mobility

Clarina is completely immobile whilst shooting her ult.
She can turn freely, she just can't move.
You can dodge to exit her ult early, but again - her dodge is slower to get back into the action when compared to other operatives standard dodges.
Clarina also can't be interrupted whilst shooting her ult.
Much like Bubu - The Geomancer, holding down a movement key will cause Clarina to dart in the indicated direction after each shot of her ult, but the overall mobility that gives is still terrible.

Healing

Each shot also heals Clarina by 25x Clarina's Lvl + 198 = 2,198
This gives 10,990 healing over the max of 5 shots.
With M4, this is increased to 27.5x +220, which gives a total of 5* 2,420 = 12,100
This sounds like a lot, but given that Clarina is completely immobile during her ult, she does really want some healing to counter the fact that she'll probably take some damage during her ult, as she's unable to move out of the way of enemy attacks.

Support Skill

Clarina deals Kinetic damage as well as a 3s AoE stun., but like most Main-DPS-ers you really don't want to use her as a support operative.

Deiwos and Passive Effect

Clarina's Deiwos gives her +1 Bonus Max HP for each 10 HP she has restored via any means (*cough* Eatchel *cough*)
For each 100 Bonus Max HP she has from this effect, she gets +10% Final Active Skill DMG.
Clarina loses 30% of this Bonus Max HP each second.
The huge 30% loss/s makes high Bonus HP amounts difficult to sustain, but Clarina restores so much HP that getting decent Skill DMG bonuses from it isn't that hard.
The bonus damage from being healed makes Eatchel and QQ much more attractive as support operatives.

Doing the maths, if Clarina did have 3000 bonus Max HP, then she would lose 30% of that = 900 per second, meaning she'd need to gain 900 bonus Max HP to sustain the max bonus of her Deiwos. Since she gains 1 bonus Max HP per 10 healing received, to maintain her max Deiwos bonus Clarina needs to receive 9,000 healing per second.
This is not feasible for standard gameplay.
...which is probably for the best, as 3000 bonus Max HP would give Clarina a colossally gargantuan +300% Final Active Skill DMG increase.
That wouldn't even be powercreep, it would just be Clarina being the meta and every other main DPS going and sitting in the corner until Clarina got nerfed.
(On the plus side, the other girls would probably have some fun poking fun at how far Jade Arc has been eclipsed on the DPS ladder...)
Regardless, Clarina can't sustain her max Deiwos bonus, even with Eatchel's help.

Mathematically, Clarina's Skill DMG bonus will asymptote towards (Healing/s * 0.03)

With Eatchel at high investment, you should expect to be able to sustain around +65-75% Skill DMG from Clarina's and Eatchel's healing.
With Eatchel and Clarina at max investment, I calculated Clarina's Deiwos should average 80.3% Final Skill DMG bonus.
Note the use of the term average in that statement.
It's a strong bonus... but Clarina was designed and balanced around having this bonus at a reasonable level, so whilst it is a large bonus it doesn't make her overpowered.

Instead, what you really need to consider is how much Eatchel provides to Clarina on her own - as Clarina will always get the bonus she gives herself.
If your Eatchel+Blitzing Fangs provides, say for example 500 HP/s, then Eatchel will be able to provide Clarina with an extra 15% Final Skill DMG.
15% is quite good just from Eatchel's healing (as that's ignoring Blitzing Fangs' actual ATK boost; +1,080 ATK for T1 and +1,800 ATK for T2).
In practice Eatchel can provide more HP than that - 7-800/s isn't overly difficult.
Eatchel thus provides the strong ATK boost she always provides, as well as a good Final Skill DMG boost, which have perfect 100% uptime.

There is a max bonus of 3000 Max HP, meaning that even if places like Paradoxical Labyrinth - Battlefield Sigma, Clarina can't just scale her damage infinitely by having stupidly high healing - whilst the limit is high, there is still a limit.

Weapon

Rosewing
Her 5* weapon, and her best weapon.
Kinetic DMG + 18/30%.
When the user consumes their HP, Skill DMG + 36/60% for 10s
When the user uses an Active Skill to restore their own HP: increases Skill DMG taken by the target by 18/30%
As always, increasing damage taken is a very rare and thus strong effect, and makes her 5* weapon much better than her 4* options.
Blight Briar
Her 4* event weapon.
When the user uses an Active Skill to restore their own HP: increases Kinetic DMG by 9/15% [T1/T5]
When the user uses an Active Skill to deal Kinetic DMG: increases ATK by 9/15% [T1/T5]
When the user has both effects, double their boosts - i.e. +18/30% Kinetic DMG and +18/30% ATK [T1/T5]

All of the effects from her 5* weapon and her 4* event weapon are basically permanent effects if playing Clarina optimally.

For weapon Parts just go with max ATK.

Logistics

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

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

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

Since all of Clarina's damage comes from her skills, and skills can't crit, crit mods are useless to her.
Clarina doesn't care for S-Energy or U-Energy, so energy recovery mods are useless to her.

+Healing only affects how much healing Clarina gets from her ult. She does benefit from it, but it is a cope stat and is very bad for her.
Clarina's own healing from her Standard Skill isn't affected by any healing modifiers, so +Healing doesn't affect her standard skill.
Note the +Healing Special stat is +Healing done, not received, so Clarina having +Healing doesn't affect how much she gets from Eatchel either.

Manifestation

M1 gives +25% Kinetic DMG for 3 seconds whenever her HP changes.
Since everything Clarina does either heals or damages her, this is basically a permanent +25% damage buff, which is great.
M2 gives her Standard Skill and Ultimate Skill 15% resistance pen.
This is a nice damage buff, but note that it doesn't affect her dodge-shot.
Since you ideally shouldn't be using her dodge shot, this caveat doesn't really matter.
M3 gives a 20% Final damage buff to her Ultimate if you have at least 50% Crimson Charge when you use her ult.
Since you should always be using her ult at max Crimson Charge, this is just a 20% Final DMG buff to her ult, which is great as her ult is a huge part of Clarina's damage.
M4 gives her ult more damage.
Without M4, her ult does: (350% + 164)*3 + (750% + 245) + (1150% + 903) = 2,950% + 1,640 damage
With M4, her ult does (385% + 205)*3 + (825% + 411) + (1265% + 1742) = 3,245% + 2,768 damage
This translates to around an 11% damage boost.
The weaker Clarina is, the more the flat bonus will count for, and M4 might be up to a 15% damage boost for a very weak Clarina.
Regardless of how weak your Clarina is, M4 is a 10-15% bonus, and is just a damage bonus.
M5 gives Clarina +25% ATK for 2 sec after she receives healing from any source.
Clarina can generally sustain this herself without any issue, but with Eatchel this is a 100% guaranteed permanent effect.

For max damage, you'll want M5.
If you want efficiency (good damage without massive investment), then M3 is a good stopping point.

Good Supports for Clarina - Crimson Moon

Clarina has two ideal supports:

Eatchel - The Cub
A Blitzing Fangs Eatchel provides Clarina with bonus ATK, which is nice.
Eatchel herself also provides healing which also provides Skill DMG via Clarina's Deiwos.
How much Skill DMG she gives varies depending on how much healing your Eatchel gives, but probably around 20-30%.
Both of these effects are entirely passive, meaning Eatchel doesn't get in the way of Clarina's damage rotation, and Eatchel's buffs have literally 100% uptime.
This makes Eatchel a very strong support for Clarina.
You do really want Eatchel to have Blitzing Fangs though, as without it Eatchel isn't really any stronger than many other support operative options for Clarina.
Eatchel is still easier to use though as she's an entirely passive buffer.
Vidya - Celestial Swan
Celestial Swan reduces the enemies resistance to help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off-element to Clarina.

Clarina's best supports are Eatchel - The Cub and Vidya - Celestial Swan by a notable margin.

Note that Clarina - Crimson Moon, Eatchel - The Cub and Vidya - Celestial Swan are all operatives that don't care for U-Energy at all - Clarina literally can't use it, and Vidya gains very little benefit from it.
This means that you will want to use your U-Energy for Eatchel's ult whenever possible, which will give Eatchel more Icebound Claws and thus more passive ATK buffing.
Since this is basically the only team composition where Eatchel is the only character that can actually use U-Energy, this also makes this one of the only team compositions where Eatchel actually gains a benefit from being fully invested all the way to Manifestation 5.

If you don't have Eatchel or Celestial Swan then consider:

Siris - Freefin
Freefin provides strong skill damage buffs, although Clarina can't make use of Freefin's Wavehunt thermal skill damage buff.
Freefin is off-element to Clarina, but Clarina can sustain Siri' buffs, meaning you only need to use Siris once, and hence she does generally work with Celestial Swan.
Marian - Riptide
Clarina doesn't inflict any "Kinetic Damage Taken" debuffs on enemies, meaning Marian isn't missing out on any damage-increasing effects due to being off-element.
Riptide provides her usual +Skill DMG and resistance-pen buffs, which is great as that's fantastic for Clarina.
Being off-element means that Riptide is not a good choice if using Clarina with Celestial Swan, but Clarina + Eatchel + Riptide is a strong team.
Acacia - Kaguya
Kaguya is a good resistance debuffer - not nearly as strong as Vidya - Celestial Swan, but if Vidya is busy that week you can use Kaguya instead.

Cope supports:

Mauxir - Shadow Ka
The utility Mauxir offers Clarina is basically zero - Clarina just doesn't need help hitting her mark for her entire rotation.
Mauxir's target dummy is still useful, however Clarina's auto-aim for her skills means that Mauxir is only useful as a minor DPS boost.
Chenxing - The Observer
At M3 Chenxing provides a 30% Skill DMG buff, as well as a +10% Standard Skill Dmg buff, which is very useful for Clarina.
The necessity to be near her supports drop pod makes her annoying to use as a support, but she is good if you can do it.
The extra range from Chenxing's M5 helps with this.
Chenxing also heals Clarina, which is nice as that gives Clarina even more Skill DMG via Clarina's Deiwos.

Other Notes:

Clarina can't use U-Energy, and hence buffs to U-Energy are useless to her, meaning U-Energy supports like Tess - The Magician and Fritia - Hush are useless to Clarina.
Whilst all of Clarina's damage does come in the form of a projectile (almost all of which come from her gun) her damage is entirely skill damage and isn't ballistic damage, meaning that it doesn't benefit from ballistic damage buffs and doesn't count as a shot hit.
This makes common gun/ballistic supports like Tess - Phantasia and Fritia - Little Sunshine utterly useless for Clarina.
Since Clarina never uses her gun, reloading and magazine bonuses are also useless to her, meaning Fenny - Starshine is useless as a support for Clarina
Similarly, Clarina never actually shoots anything, meaning you have to interrupt Clarina's rotation to actually shoot normally to gain the full benefits from Enya- Exuvia, making her a terrible support choice for Clarina.

As a reminder, Clarina might visually appear to shoot things with her gun, but she deals entirely skill damage, and does not deal ballistic damage.
She is also unaffected by ballistic buffs, as shown here:

Lyfe IS shoots stuff, and thus benefits from aptitude/per-shot bonuses:
White numbers are Lyfe IS' own Kinetic DMG
Blue numbers are from Tess - Phantasia's per-shot Frost damage buff
Red Numbers are from Fritia - Little Sunshine's per-shot Thermal damage buff

Lyfe IS can benefit from useful ballistic, per-shot, and shot-hit buffs.

Clarina's skills don't technically shoot stuff.
Hence, she can't gain or use ballistic, per-shot, or shot-hit buffs.

The extra Frost/blue and Red/Thermal damage numbers Lyfe IS gets in the picture to the left aren't present here, as Clarina isn't technically doing any shot hits to proc the buffs, even though the support buffs are active.
(The green numbers are her self-healing. Yes I was testing her at Lvl.75, hence 252)
Supports like Tess - Phantasia and Fritia - Little Sunshine are thus worthless to Clarina.