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 (Chapter 13 - 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 require constant button clicking to frequently use her support skill.

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 means she can feel like a more mobile version of Fritia - Hush.
Starshines clothing also gets more revealing when she has a special mag (this is true of all of her outfits, not just the default).

Overall

Role: Ballistic Support
Deals Kinetic Damage
(Although ideally doesn't deal damage)
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 standard skill and magdump
Against Bosses: Use her standard skill to do a sideways-dash standard skill and magdump

How to use 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 or +19.3 Skill Haste
+8.5% Electric Damage
Best teammates:
Any operative that deals ballistic damage
(The larger their magazine capacity the better)

Standard Skill
Starshine's standard skill causes her to dash. If you're already moving, she'll dash in that direction. Otherwise she'll dash 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 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.
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.

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 enemy resists, which is strong.
The special clips also give extra 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% for 5s. The damage bonus doulbes 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.
Strawberry Shortcake can also be easier to get than Pixel Era, as you can get it from any weapon banner.

For weapon Parts, just go with maximum ATK.

Logistics

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 is using a Clip from her standard skill.

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 useful to her.

For the basic talent, you want +10.0% ATK and either +106 Alignment Index or +19.3 Skill Haste.

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.
Skill Haste will allow Starshine to recharge her special Clips quicker, which can be more useful for longer combat.
If Starshine has three Skill Haste mods then she will be using slightly more S-Energy than she is using (10 S-Energy used every 9.49s) but combat will basically never last long enough for that to be an issue.

Alignment gives slightly more damage, but over longer fights where more Clips are needed Skill Haste can be much better.
If you have Starshine you can simply use her and see if you want the extra Skill Haste, and if not then go Alignment.

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, and increases the ATK bonus from 20% to 22%.
Around a 10% buff, but most of Starshines utility comes from her weapon/logistics buffs and not needing to reload, so the actual benefit you get from M4 is quite small.
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 the more frequently Starshines support skill needs to be used.
For operatives with Assault Rifles, SMGs, Dual SMGs, or Crossbows this isn't 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 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 with all ballistic operatives, in practice she doesn't really work overly well with pistol or sniper operatives.