Siris - Freefin

Siris - Freefin is not currently available as a Trial Operative in any story mission, so if you don't have her yet the only way to try her is to wait for her banner so she has a trial.

As a support, Freefin can provide strong and potentially permanent skill-damage buffs that are very useful for any operative that frequently uses their skills.
Freefin does provide a Thermal-damage skill buff, and hence works a bit better for operatives that deal thermal skill damage.
Freefin's Support Skill also only deals damage once upon use, and can be maintained indefinitely, allowing Freefin to work alongside Vidya - Celestial Swan.

As a DPS Freefin is ok but not great, and the Sub-DPS role provided by her ult doesn't perfectly fit alongside any current main-dps operative, meaning that when using her as a support, Freefin's ult is something you ideally never use.
Despite this, Freefin has fantastic mobility, and can hit multiple targets with her PBAoE attack, and her very high HP and mobility can give her very good survivability too, although her PBAoE attacks means her ds against distant or flying targets is abysmal, and her PBAoE attacks mean she can end up taking a lot of damage due to her proximity to enemies.

The Basics

Role: Support - Skill-damage
Deals Thermal Damage
Deals entirely Skill dmg
Mobility is incredible
Ult generates quite a lot of shield

This is Freefin's "Freetide" gauge.
Freetide increases Skill Damage.
Max Freetide is 20.

How to use Siris - Freefin as a Support

Use her Support Skill to buff the deployed operatives skill damage.

It ideally shouldn't, but if her Support Skill does run out, re-apply it again immediately off-cooldown.

How to use Siris - Freefin as a DPS

Use her standard Skill to get on her jetboard and enter Flowride state
Whilst on her jetboard, use her attack to deal PBAoE damage
This also charges her Swell Gauge.
If her Swell gauge is full, use her Standard Skill to deal stronger PBAoE damage.
(These stronger attacks are where most of her damage comes from)
If her ult is ready, use it to deal damage and get on her jetboard to enter Flowride state, and then gain Swell Gauge at a higher rate.

How to build Siris - Freefin

Weapon: Splash Glider / Ancient Nautilus
Weapon Parts: Max HP
Logistics Squad: Libra
Basic Talent: +10.0% Max HP
+106 Alignment Index
Special Talent: +8.5% Thermal Damage
Best teammates for Freefin to Support:
Any operative that deals Skill damage.
(Works slightly better with Thermal operatives)

Siris - Freefin's "Swell Gauge" indicator

This bar is Freefin's "Swell Gauge".
Max Swell Gauge is 100.

Specifics:

Standard Skill

Using her standard skill causes Siris to ride her jetboard around, which the game calls her being in the "Flowride" state.
Whilst riding she doesn't naturally recover S-Energy, but with her Neuronic she gains 5 S-Energy each time she hits an enemy, allowing her to easily maintain it in combat.
She also loses 5 S-Energy each second to increase her speed by 20%.
This is an additive buff, not multiplicative, but either way it's not something that actually makes any difference to how she is played.

Whilst in Flowride, Siris can't shoot, but instead does a "Surfer Swirl" that deals PBAoE damage to up to 4 nearby enemies and grants some U-Energy.
This also grants Siris 15 Swell Gauge per target hit.
Siris has a maximum Swell Gauge of 100.
Siris' Swell Gauge is the bar in the middle-right of the screen.
Using her standard skill again whilst already in Flowride will do a "Tidal Strike" which will use all Swell Gauge to deal PBAoE damage.
The more Swell Gauge she uses, the more damage a Tidal Strike deals.
With her Neuronic, Siris restores 25 S-Energy with her Tidal Strike, which makes is trivially easy to sustain her Standard Skill in combat.
With her other Neuronic, using Tidal Strike at the max of 100 Swell Gauge causes it to deal +20% Final DMG.
Her Tidal Strikes are where the vast majority of Freefin's damage comes from.

Using a Tidal Strike while at max Swell Gauge doesn't exit Flowride and instead resets her standard skill cooldown.
If Freefin doesn't have max Swell Gauge when using Tidal Strike, then she exits Flowride afterwards.
Dodging also exits Flowride.

The damage dealt by this skill scales with Siris' Max HP, not her ATK.

In essence, use her standard skill to enter Freeflow, and then while in Freeflow use her Surfer Swirl (i.e. LMB attack) until her Swell Gauge is full, and then use her standard skill to cast a Tidal Strike.

Ultimate Skill

Siris creates a whirlpool that deals AoE damage up to 10 times and knocks targets up into the air.
The whirlpool then explodes, dealing Thermal damage and causing Siris to enter the Freeflow state of her Standard Skill.

After using her ult, Freefin enters "Tsunami" state for 12s, which gives her an additional 3 Swell Gauge every 0.1s.
This means she gains 30 Swell Gauge every second, or 360 Swell Gauge total, which is enough for 3.6 fully-charged casts of her standard skill.

If used at max Swell Gauge, Freefin's standard skill detonation instantly recharges the standard skill, allowing it to be quickly spammed.
This means that if you don't wait for her Swell Gauge to be full before unleashing her standard skill, then it will go on cooldown, which will waste a lot of the Tsunami state's Swell Gauge generation.

Freefin also grants the entire squad a shield whilst she is in Tsunami state. The shield is removed when Tsunami ends.
With her Neuronic, hitting enemies with her standard skills Tidal Strike generates "Gleamscale" that you can collect for more shield.
This shield is based on Siris' Max HP, and since she has to much Max HP, her shields can actually provide a lot of survivability.

With her other Neuronic, dealing damage with her standard skills tidal strike extends Tsunami, up to a max total duration of 18 seconds.

Note that Freefins ult uses 60 U-Energy.
This will be notable when discussing her relationship with Fenny - Pureheart in the teammates section later on.

TL:DR: Her ult deals PBAoE damage, and causes her to enter her standard skill state and charge it very quickly.
After using her ult, make sure her Swell Gauge is full before using her standard skill so you don't waste it.

Passive Effect

If Freefin has a Thermal weapon equipped, then whenever the deployed operatives skill hits a target, Freefin grants them 3 Freetide stacks, gained at most every 0.5s.

Each stack of Freetide gives +1% Skill Damage, plus 0.1% per 100 Alignment Index.
If an operative doesn't gain a stack for 1 second, then they instead lose a stack of Freetide.
The little fish icon on the middle-left of the screen shows you how many Freetide stacks you have.

In essence, operatives that can frequently damage enemies with their skills get a nice skill damage buff from having Freefin on the team.

Support Skill

Freefin deals Thermal Damage to the target, and also decreases its ATK by 10% for 5s.
With her Neuronic, an operative with Wavehunt also re-applies the -10% ATK debuff, allowing it basically 100% uptime even without spamming her support skill

Freefin's Support Skill also gives the deployed operative 20 Freetide stacks and 5 Wavehunt stacks.

Whilst active, her Support Skill generates 3 additional Wavehunt stacks when dealing skill damage, up to every 0.5s, up to a max of 20 stacks.
Wavehunt stacks increase Thermal skill damage by 1% per stack.
Having at least 10 stacks also gives an ATK boost of 1% of Freefin's Max HP. (Maybe 400 ATK with low investment. 500-600 with high investment.)
Having any Wavehunt stacks means that Freetide stacks don't disappear, allowing operatives to stay at max Freetide stacks for a strong skill damage bonus.
If Freefin has a Thermal weapon and her Neuronic, then she also gives an extra 2 Wavehunt each second for 5s after using her support skill, for 10 total extra Wavehunt.
If a new stack isn't added every second, one stack is removed instead.
All stacks are lost when leaving the field.

Each Freetide Stack grants 0.25-1% Skill Damage (depending on her Deiwos), plus 0.1% per 100 Alignment Index.
For the usual max of 618 Alignment Index, this gives up to +1.618% Skill Damage per Freetide stack.
For the 20 stacks her Support Skill gives and allows her to maintain, this is between 5% and 32.36% Skill Damage, depending on how much Alignment Index and Deiwos she has.

This means that using Freefins Support Skill grants an immediate Skill Damage boost, and a stacking Thermal Skill Damage Boost that increases over time up to a decent maximum, provided you regularly deal skill damage.

The TL:DR is that you can and should just use her support skill, and provided you are dealing skill damage decently often, Freefin's buffs will just maintain themselves without needing to re-cast it.
If your dps doesn't deal skill damage often enough, then Freefin's buffs will eventually expire, in which case you can just spam it whenever it comes off-cooldown, but if your dps can't sustain Freefin's buffs then Freefin is likely not as good of a support for that dps, as the strength of Freefin's buffs scale with the number of stacks she has.

Weapon

Splash Glider
Gives 21-35% Skill DMG whilst Freefin is off the field. After using Freefin's support skill, Freefin also gives the deployed operative a +15-25% ATK buff.
Whilst both effects are active, enemies damaged by the operatives also take 12-20% more skill damage
It gives Siris herself a 21-35% thermal skill damage buff whilst she's on the field.
It also gives 18-30% Max HP, improving Freefin's damage and the strength of her ATK buff.
Ancient Nautilus
Gives a +24-40% Skill DMG bonus after using Siris' support skill, which lasts until Siris' is deployed to the field, which is a very nice support buff for a 4* weapon.
It also gives 12-20% Max HP, which is very nice.

Her weapons aren't that strong for improving her own damage, but they are excellent for supporting other operatives, which is her main role.
They also only give generic skill damage and ATK buffs, meaning she is useful for non-thermal operatives just fine.
They also give a Max HP boost as their passive effect, which is very useful for Siris.

If you don't have either Splash Glider or Ancient Nautilus, then consider

Pine Aurora
The signature 5* of Acacia - Kaguya.
The Alignment Index bonus (+549 when maxed) this gives increases Siris' Skill Damage buffs very nicely.
Tiny Tool
Gives an Alignment Index buff (of +413 when maxed), which is also very useful
Abyssal Light
The signature weapon of Enya - Exuvia.
If Siris is supporting a skill-damage operative that deals Electrical damage, this is an ok option.
Prismatic Igniter
Gives a 15s ATK buff after using her support skill, based on Siris' ATK.
Since Siris doesn't have much ATK, and her support skill lasts indefinitely and thus doesn't need to be re-cast, this is only good in very short fights.
If you have one and don't have anything else for Siris though it's her best choice.

For weapon parts Freefin wants Max HP, as all her damage scales off of her HP, and not her ATK.

Logistics

Squad

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

Libra squad provides the entire team with a 30% Skill DMG buff whilst Freefin is off-field, which is a nice buff.
Libra also gives a +25% ATK buff the the deployed operative with Freetide and Wavehunt, which is a strong buff whils Freefin's support skill is active.
Siris also gains 3 extra Freetide each second whilst deployed, although as she's primarily a support that's not the most useful buff.

Note that whilst Freefins support skill does have a 5s cooldown, it only goes on cooldown after all Wavehunt stacks have expired, meaning that in practice the uptime is high and the time between casts is much higher than 5s. As such Twilight squad is an extremely bad choice for Siris.

Stats

Freefin wants:

+10.0% Max HP - To boost her damage and her Wavehunt's ATK bonus
+106 Alignment Index - To boost the skill damage buff from her Freetide stacks
For the Special talent, Freefin ideally wants +8.5% Thermal Damage, but since you should ideally maintain her support buff indefinitely, Freefin ideally only deals damage once per fight, making the Thermal damage buff basically useless.
+Thermal damage is nice to have as her 3rd stat, but it makes so little difference you shouldn't waste re-roll tokens to get it.

Since she ideally doesn't need to use her support skill more than once per fight, Skill Haste is useless.
All her damage and buffs scale off of her Max HP, so +ATK is useless to her.
S-Energy and U-Energy are not useful to her as a support.
If using Freefin as a main dps, then U-Energy recovery is a decent cope stat.
Since Siris only deals skill damage, and skills can't crit, crit buffs are useless to Freefin.

Shield Construction Efficiency is a very bad cope stat of you are using Freefin on-field, but if you are only using her as an off-field support it's literally useless.

Manifestation

M1 gives her Support Skill 20 Wavehunt rather than the initial 5 stacks.
This gives max wavehunt up-front, and thus gives the maximum thermal damage buff immediately.
It also allows operatives who deal skill damage infrequently enough that they are unable to maintain Freefin's support buffs a longer uptime on the buffs.
M1 also gives +5% Final Skill DMG when at 10 or more Wavehunt, which most skill damage operatives can maintain.
Even if an operative can't maintain it, since they now start with 20 stacks they'll get some decent uptime on it anway.
+5% Final DMG is decent, but not that strong, and it doesn't change how Freefin plays.
With M2, any operative with Wavehunt ignores 10% All-Type Resistance and 5% Thermal Resistance.
This is good for all operatives, but obviously a stronger buff for thermal-damage operatives.
M3 causes targets with a Wipeout mark to take 12% more skill damage.
Given how rare "damage taken" mods are, this is very strong.
M4 gives the usual +1 skill level, increasing the strength of the buffs that Wavehunt gives by 10%, which is very nice.
Note that this buffs her Wavehunt's Thermal-damage buff and ATK boost only.
Operatives that both <don't deal Thermal-damage> and <scale their damage off of their Max HP and not ATK> won't gain any benefit from Siris' M4.
Operatives that have only have one of the above apply will get much less benefit out of Siris' M4 than operatives who can use both the thermal and ATK buffs.
M5 just gives +15% ATK when at 10 or more Wavehunt stacks, which is a simple buff that's pretty easy to maintain.

If you want efficiency (good damage with minimal investment), then M3 is a good stopping point.

Good teammates for Siris - Freefin

Freefin works well with any operative that deals skill damage.

Freefin works better with operatives who can maintain at least one wavehunt stack, however almost all skill-damage operatives can.

Freefin works best with operatives who deal thermal skill damage, but the difference is small, although it gets larger at M2.

Since Freefin ideally only deals damage when she first uses her support skill, and ideally only does that once per fight, that means she also works perfectly with Vidya - Celestial Swan, even for operatives that don't deal thermal damage.

The best operatives for Freefin to support are:

Siris - Ksana deals exclusively Thermal Skill Damage and can trivially maintain max Wavehunt stacks, making Freefin an ideal support for her.
Fenny - Pureheart deals almost entirely thermal skill damage and can maintain max Wavehunt, and hence works almost perfectly with Freefin.
Fritia - Hush spends most of her time in combat shooting stuff, but most of her damage is in her ult which works well with Freefin as Hush' ult is entirely thermal skill damage.
Nita - Pulse deals exclusively Thermal Skill Damage, and can use skills extremely frequently, making her a great dps to pair with Freefin, however since Pulse' damage scales with Max HP, Freefin's ATK buffs (Wavehunt, logistics set, 5* weapon) are wasted on Pulse. Freefin is still a very good support for Pulse, but their synergy isn't quite perfect.

Note that Siris' Ultimate isn't worth using with any current dps operative, as there is no scenario where you would want to swap out your main DPS to have Freefin swap in and start doing damage - you're always better off just maintaining Freefin's support skill and having it buff the damage of your main DPS.

Further note that Freefin's ultimate cost 60 U-Energy, which is more than Fenny - Pureheart's ult uses:

If having Freefin support Pureheart whilst Pureheart uses her LMB combo, you'd need to not use Pureheart's ult for a while to gather enough U-Energy to swap Freefin in via her ult, which is a large loss of dps for Pureheart.
Siimilarly, Pureheart offers absolutely no support buffs to Freefin at all, so whilst Freefin is on the field Pureheart is basically just dead weight on your team.
Pureheart also just deals more dps than Freefin anyway, so even ignoring the buffs Freefin can provide Pureheart, you still wouldn't want to swap Freefin to use her ult.
If you are using Pureheart as your dps and using Purehearts LMB chain, then it's never worth it to swap Freefin in via her ult - Freefin deals less dps than Pureheart, and Pureheart can't support Freefin, but Freefin's buffs are good for Pureheart, who is the better dps.
Since Pureheart's RMB attack doesn't allow Pureheart to use her ult, any U-Energy Pureheart gains whilst spamming her RMB combo is useless for herself as she can't use her ult, and thus allowing Freefin to use it for her ult does work, however since Freefin isn't really any better at AoE than Purehearts RMB attack is, you really wouldn't need or want to ever do this anyway.
In addition, Pureheart's RMB attack also doesn't generate any U-Energy, so you wouldn't be able to get enough U-Energy to swap Freefin in via her ult to begin with anyway...

As such, I recommend that if you are using Freefin as a support for another dps, then you should never use Freefin's ultimate ability.