Fenny - Starshine

If you don't have her yet but want to try her out, Starshine is available as a Trial Operative in Story Mission 13-12 (Legacy // New Hentiro // Suspense in Skytopia)

As a support, Starshine provides magazines to your main dps, meaning they don't have to reload, whilst also providing good ballistic damage buffs, which make Starshine a strong ballistic support, albeit one that can potentially require constant button clicking to frequently use her support skill.
Operatives that empty their magazines quickly can very quickly drain Starshine though, meaning she doesn't work very well with some ballistic operatives.

When used as a main dps, Starshine has high mobility and decent damage [for a support].
Spending most of her time in combat shooting, with an ult that fires a giant laserbeam, does mean she can feel like a more mobile version of Fritia - Hush.

Starshine's clothing also gets more revealing when she has a special mag (this is true of all of her outfits, not just the default one).

Overall

Role: Ballistic Support
Deals Kinetic Damage
Weapon Type: Assault Rifle
Deals mostly ballistic damage
Mobility is very good

How to use Fenny - Starshine

As a support:

If your dps doesn't currently have a special clip from Starshine, use Starshine's support skill to give your dps a fresh mag

As a DPS:

If Starshine has run out of S-Energy then use her standard skill to backwards-dash and magdump.

Otherwise:

Against Groups: Use her standard skill to do a forward-dash and magdump
Against Bosses: Use her standard skill to do a sideways-dash and magdump

How to build Fenny - Starshine

Weapon: Heart Hunter / Pixel Era
(Strawberry Shortcake is also decent)
Weapon Parts: Max ATK
Logistics Squad: Reverie
+10.0% ATK
+106 Alignment Index
+8.5% Kinetic Damage
Best teammates:
Any operative that deals ballistic damage
(The larger their magazine capacity, the better)
(The lower their RoF, the better)

Specifics

Standard Skill

Starshine's standard skill causes her to dash. If you're already moving, she'll dash in that direction. Otherwise she'll default to dashing backwards.
Starshine will give herself a "Stargem Clip" depending on which direction she dashes in:
Forwards: Uses 15 S-Energy RoF/3, Final damage doubled, shots deal AoE damage to up to 3 targets, but can't crit.
The forward dash gives 1/3rd the bullets of the other two dash directions.
Left/Right: Uses 15 S-Energy Shots deal 20% increased kinetic damage, but are otherwise normal.
Backwards: Doesn't use S-Energy Shots restore 1 S-Energy upon hit, but are otherwise normal.
If Fenny doesn't have enough S-Energy for a forward/sideways dash, she will consume all her S-Energy to give herself bullets proportional to how much S-Energy she consumed.

These are a special magazine, and bullets from them are used before her actual magazine.
The bullets from these Clips are special and do not work with any aptitude effects, such as Tess - Phantasia or Fritia - Little Sunshine.

As such, you can dash forwards/sideways in order to get shots that deal more damage, then when Fenny runs out of S-Energy, dash backwards and shoot to quickly restore S-Energy.
Use her forward dash against groups, and her sideways dash against bosses or other single targets.

Ultimate Skill

Starshine fires a giant laser beam that deals kinetic damage every 0.2s for 5s (for 25 hits total).
While it's firing, Fenny also takes 50% less Final damage.

With its first Neuronic, it also gives Fenny some S-Energy back, allowing her to do her forward/sideways dashes more often, for higher dps.

With its second Neuronic, the damage is buffed by generic ballistic damage (But not ADS or hipfire ballistic buffs specifically, only generic ballistic damage)

The total damage is pretty good, but its high U-Energy cost of 80 means the dps isn't brilliant.

Support Skill

Starshine gives the deployed operative a "Starstone Clip".
This is a special magazine, and bullets from it are used before your operatives actual magazine.
Bullets from the Starstone Clip give an extra 20% of Fenny's ATK.

This skill has a 15s cooldown, but holds up to 2 charges (3 with its Neuronic) which is generally enough for shorter fights (or even mid-long fights if you have enough mag size).
Provided she has enough charges, you can used this skill repeatedly to give your operative fresh magazines back-to-back, meaning they don't have to reload.

The bullets from Starshine's Starstone Clips are special and do not work with any aptitude effects, such as those from Tess - Phantasia or Fritia - Little Sunshine.
Similarly, operatives can only have one type of Special Clip at a time, so operatives like Mia - Starchaser who always have another non-Fenny Special Clip won't work with Starshine

This skills first Neuronic can reduce its cooldown by 1s each time you deal a crit or hit a weakspot, triggering up to once per second.
This allows you to potentially reduce the cooldown from 15s down to 7.5s.
Whilst this is good, when supporting operatives with small magazines (e.g. Bubu - The Geomancer) Starshine can still run out of charges somewhat quickly.
Needing to spam her support skill very frequently to support operatives like Bubu can also mean that Fenny can run out of S-Energy quite quickly as well.
It can also make Starshine very tedious to play.

Weapon

Heart Hunter
Her 5* weapon, and her best weapon.
This gives more ATK, and shots from any of Starshine's special Clips to ignore 12-20% of the enemies Resistance, which is strong.
The special clips also give +15-25% ballistic damage (which doubles if Starshine is off-field).
Pixel Era
When Starshine provides a special clip, increases the deployed operatives ballistic damage by +7.5-12.5% for 5s. The damage bonus doubles if Starshine is off-field.
The buff is quite short, but Starshine can provides mags very quickly, sustaining the bonus in combat is not overly difficult.
Strawberry Shortcake
This gives an ATK buff for 15s after using a support skill.
Pixel Era generally outperforms Strawberry Shortcake if you can maintain high uptime on Starshine's mags, but Strawberry Shortcake is still a good option.
If you can't maintain high uptime on Starshine's mags, then you probably shouldn't use Starshine, but if you did then Strawberry Shortcake will outperform Pixel Era.
Strawberry Shortcake can also be easier to get than Pixel Era, as you can get it from any weapon banner, or even the regular Shop.

For weapon Parts, just go with maximum ATK.

Logistics

Squads:

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

Reverie squad gives +40% Ballistic damage to any operative that has one of Starshine's special Clips, which is very good.
It also increases Starshine's ATK by 20% when she herself is using a Clip from her standard skill.

Stats:

For the Special talent, Starshine wants +8.5% Kinetic Damage, although if you are using her as an off-field support you can basically ignore the special talent as none of them are are actually useful to her.

For the basic talent, you want +10.0% ATK and one of: +106 Alignment Index, +19.3 Skill Haste, or S-Energy regen.

ATK increases the extra damage her support skills special Clips do.

Alignment Index increases the full squads ballistic damage. +1% Ballistic damage per 100 Alignment Index isn't much, but it's something.

If your dps operative has a small magazine then Fenny can run out of charges for her support skill, or potentially run out of S-Energy.
Skill Haste and S-Energy Regen can help with that, and in those cases those stats are great for Fenny.
I'd generally recommend Alignment Index though - it is a useful buff for all the ballistic operatives you'd actually want to have Starshine support, whereas Skill Haste and S-Energy Regen are more niche and potentially situational.
Skill Haste and S-Energy can be great picks, but only if your dps actually needs them, and can be useless in many situations.

Manifestation

With M1 and a Kinetic Weapon equipped, Starshine gives extra effects to operatives with one of her special Clips
Sniper, Pistol, and Crossbow: Increases Final DMG dealt to weakspots by 15%. This is a huge damage buff.
Shotgun: +1 Pellet. Since shotguns typically have 8 pellets, this is a flat +12.5% total damage buff, which is very good.
AR, SMG, and Dual SMG: Each shot has a 12% chance to fire an extra bullet, without using any extra ammo. This is basically a 12% total damage buff.
Starshines M1 is very strong
M2 increases the capacity of the Clips given by her support skill by about 20%
This allows them to last longer, and means you need to spam Starshines support skill less, which is a nice QoL buff.
For short fights where only a few mags are needed M2 doesn't increase damage in any way, although the QoL is still nice.
M3 decreases her support skills cooldown to 12s, and its S-Energy cost to 8
This increases the rate at which she can generate extra mags, and the reduced S-Energy cost helps her sustain over longer fights.
For short fights where only a few mags are needed, M3 doesn't help in any way.
M4 gives her support skill +1 level, increasing the ATK bonus from 20% to 22%.
The actual damage buff increases by around 10%.
A lot of Starshine's utility comes from not needing to reload though, so the actual benefit you get from M4 is smaller when compared to other supports.
M5 causes bullets from Starshine's special Clips to ignore 10% of the enemies resistance.
This is a nice damage increase.

If you want efficiency (good damage with minimal investment), then M2 is a good stopping point.
M3 is also a good stopping point if your fights tend to last a long time.

Which teammates to use with Starshine?

Starshine provides strong ballistic damage buffs, and giving out extra Clips means that operatives don't need to reload, which a utility that is appreciated by all ballistic dps operatives.
As such, Starshine works well with all operatives that deal ballistic damage.

Since you'll want to use Starshine's support skill whenever your dps operative empties their previous special Clip, the smaller the operatives magazine capacity and the higher their RoF the more frequently Starshines support skill needs to be used.
For operatives with Assault Rifles, SMGs, Dual SMGs, or Crossbows this isn't usually that much of an issue, but for operatives with pistols or sniper rifles it can mean that Starshine only gives out a few bullets per support skill use.
For example Bubu - The Geomancer has a magazine capacity of 6 or 7, meaning that in order to sustain Bubu with enough ammo, Starshine will be needing to use her support skill every 2-3 seconds or so, which Starshine can't do over even quite short fights due to her 3-charge limit [as well as the extreme amount of S-Energy that would use].
As such, whilst Starshine does technically work very well with all ballistic operatives, in practice she doesn't really work overly well with some of them.