Fritia - Little Sunshine

If you don't have her yet but want to try her out, she is available as a Trial Operative in Story Mission 1-6 or 7-11

Being a 4* operative, Little Sunshine isn't as good as the 5* operatives for endgame content and she doesn't perform very well as a main dps, but her support skill giving a per-shot aptitude buff makes her a useful support for a lot of operatives, and whilst not the strongest support she can still be a respectable support for several operatives even at endgame should you not have any good 5* supports.

The Basics

Role: Support - Extra damage per-shot
Deals Thermal Damage
Deals mostly Ballistic damage
Mobility is decent

Advantages

Quite a good support for many operatives

Disadvantages

Performs poorly as a main DPS

How to use Fritia - Little Sunshine
Use her as a support operative and spam her support skill off-cooldown.

Standard Skill
Fritia creates an AoE in front of her which deals Thermal damage and interrupts, as well as dealing Burning for 5s.
By modern dps standards it's not overly good, but it's a good ability for a 4* operative when upgraded with its Neurnics.

Ultimate Skill
Fritia creates a large AoE that continuously deals Thermal damage and inflicting Burning.
There is a limit to how often it can deal damage, so the more enemies there are the better it performs.
When her ult ends, it also deals a powerful AoE blast.

80 U-Energy cost means that the cost it high, and the damage and dps isn't great considering how infrequently you'll use it.

Support Skill
Fritia gives the deployed operative Thermal Aptitude, which gives extra Thermal damage to each shot fired.
The aptitude effect lasts 10s, on a 25s cd, or 15s cd with M1.
With her Neuronic, it applies Burning, and with the other Neuronic it also increases the Final DMG deals to Burning targets by 20%.

Whilst the extra damage-per-shot isn't overly high, it is on every shot.
This means that the more shots the operative fires, the more extra damage is gained.
As such, operatives that have an extremely high fire rate will gain more damage from Fritia's support skill.

Note that literally all of her Manifestations boost her support skill, for considerably better damage.

Weapon

As a support: Strawberry Shortcake
This gives an ATK buff to the deployed operative when the equipping operative uses her support skill.
Given that Fritia works best as a support operative, Strawberry Shortcake is her best weapon.
The buff from Strawberry Shortcake lasts 15s, which is the same as the cd of her Support Skill at M1.
As a main DPS: Lava Bones
Improves her Skill DMG to Burned targets and Ultimate Skill DMG, which is actually not the greatest buff for her.
This is the only 5* Thermal Assault Rifle, and is thus her highest-damage option due to its higher base damage, however Little Sunshine is a poor main-DPS and works best as a Support, so I don't actually recommend Lava Bones for her - Little Sunshine is best used as a support operative using Strawberry Shortcake.

For weapon Parts just go with ATK.

Logistics

For the Special talent, you want +8.5% Thermal Damage
For stats you want +10.0% ATK. For the other stat you have three real options:
Alignment Index - Alignment Index gives the full squad more dmg dealt to Burning targets.
Given that 106 Alignment Index is only 1.06% extra damage dealt, even a maxed Alignment Index roll is not a huge increase.
Skill Haste - Little Sunshine's Support Skill has a 10s duration and a 15s cooldown, so Skill Haste is useful to increase the uptime on her support skill.
With enough Skill Haste, Fritia can use her support skill every 10s, giving it 100% uptime.
S-Energy Regen - All operatives naturally regen 1 S-Energy per second. Fritia's Support skill uses 15 S-Energy and has a 15s cooldown, so she's normally fine.
If Fritia gets Skill Haste though then she will use her Support Skill more often than every 15s, meaning she is using more energy than she regens.
Thus if Fritia has some extra Skill Haste then S-Energy is useful for allowing her to use her support skill even for longer fights.

Note that Strawberry Shortcake can maintain 100% uptime even without any extra Skill Haste.

It will take Little Sunshine 80 seconds of spamming her support skill every 10s with no extra S-Energy regen to have her support skill buff run out and not have the S-Energy to be able to re-apply it.
Very few enemies in SCZ take 80 seconds, and if you've got to the point you have a logistics officer you are considering re-rolling, I doubt any fight lasts that long for you.
Realistically the only fights that take that long are Neural Sim and Supreme Battleground bosses.
As such, you can generally get away with Little Sunshine not having any extra S-Energy regen, although it's not a bad roll to get.

Skill Haste formula:
Skill Haste works as a CD/(1+X) modifier.
A max Skill Haste roll is 19.3.
19.3 Skill Haste = 83.8% of normal cooldown
38.6 Skill Haste = 72.15% of normal cooldown
57.9 Skill Haste = 63.33% of normal cooldown

Since Fritia has 66% uptime already, she needs to reduce her cd by 33% in order to have 100% uptime on her buff.
Two maxed-roll Skill Haste buffs will give her support skill a cd of 10.82 seconds, which is generally all she needs in practice.

My recommendation for stats is thus two Skill Haste rolls.
For your third roll you can take
S-Energy regen if fights consistently last a long time for you (around 2 minutes or more)
Alignment Index for just a bit more damage. Fritia won't quite have 100% uptime, but she'll be pretty close.
Skill Haste if you want Fritia to have max uptime on her buff. This is the highest-damage option.
If you do get three Skill Haste rolls then you don't need to max them - she just needs her Skill Haste rolls to add up to about 50 or more - Three 16.4 rolls is fine.

Given that Little Sunshine is a 4* operative, I don't recommend you use logistics re-rolls on her.

As for which squad to use, you will ideally want to use either Twilight or Amarna.
Twilight gives a nice resistance debuff for 5s on using her support skill, whereas Amarna gives a nice ballistic buff for 15s and an ATK buff for 1.5s upon her support skill dealing damage.
Which of these is more useful depends on what you are fighting and what other sources of resistance reduction and damage you have, as well as how much Skill Haste Fritia has.
(Given that Twilight only lasts 5s, it only has at-best 50% uptime, or 33% uptime if Fritia doesn't have any Skill Haste rolls)

In practice you might want to simply pick whichever of the two you find easier to acquire and use that - as you'll likely transfer the logistics squad from Little Sunshine over to a 5* support operative who can use it when you gain such an operative.
Since both Amarna and Twilight are both viable for some 5* support operative at endgame, both are fine for this purpose.
As such, don't worry too much about which logistics squad Fritia uses - you'll probably end up swapping them out later anyway.
Similarly, it's not worth re-rolling Little Sunshines logistics stats - those re-rolls are better used on 5* operatives.

If you are using Little Sunshine as a support because you don't have any better 5* supports or much investment into your other operatives, then you are likely not going to have much resistance reduction, in which case Twilight is likely better.
If you do have several good operatives or just lots of investment into your 4*'s, then Amarna is likely the better logistics squad for Little Sunshine, and is what I'd consider her ideal endgame set, but Twilight is still great if you already have it and want to just keep using that.

Manifestation

As a 4* operative, you'll get all of her Manifestations eventually anyway.

All of Little Sunshine's Manifestations buff her Support skill in some way.

Good Teammates for Little Sunshine

Little Sunshine gives extra Thermal damage per shot.
As such she only works with characters that shoot, and works best for characters that shoot a large number of shots, ideally also deal Thermal damage.
Again, Little Sunshine isn't the strongest support, and many 5* supports outperform her by a large margin, but if you don't have any better supports she works well with operatives that have a high fire rate, and ideally a high magazine capacity and low reload time so that even when factoring in reload time they have a high average fire rate.

Katya - Rhapsody
Rhapsody's minigun fires 30 shots per second, meaning Little Sunshine's buff adds a lot of damage.
Fritia's aptitude buff adds (7% of Fritia's ATK + 9) to each shot. To Rhapsody this is (210% of Fritia's ATK + 270) per second, which is a lot.
Katya - Blue Bolt
Much like Rhapsody, Blue Bolt fires at a very high rate, allowing for much more damage.
All Crossbows have a base Rate of Fire of 150 shots per minute, or 2.5 shots per second, allowing for Fritia's buffs to add a lot of damage.
Lyfe - Infinite Sight
Lyfe IS has a high fire rate, a large magazine, a fast reload, and doesn't spend much time using skills, allowing Fritia to add a lot of extra damage.
Katya - Dawnwing
Dawnwing has an AR with a high fire rate. Whilst Dawnwing does have to frequently stop to use her standard skill, she doesn't have to reload if played properly.
This means Dawnwing has low downtime and spends a lot of her time firing, allowing Fritia to add a lot of damage.
Lyfe - Wild Hunt
Wild Hunt has a large magazine capacity and high fire rate, again allowing Fritia to add a lot of extra damage.
Basically any operative that has a high fire rate:
Fritia - Hush
Note that Hush is on-element to Little Sunshine, and is basically the only main DPS that is both on-element and works well with Little Sunshine.
Chenxing - Ethereal Cloud
Cherno - Enigma

If you don't have any good 5* operatives (or any 5* operatives at all), then Fritia works quite well with these 4* operatives:

Lyfe - Wednesday
Wednesday has a high magazine capacity and a very high fire rate, allowing Fritia to add a lot of damage to her.
Chenxing - The Observer
Chenxing's AR has a high fire rate
Mauxir - Meow
Again, high fire rate and large magazine capacity
Cherno - Those Two
Also high fire rate and large magazine capacity