Enya - Big Sis

If you don't have her yet but want to try her out, she is available as a Trial Operative in Story Mission 5-12

Big Sis deals low dps, is clunky, often inaccurate, and doesn't have strong mobility or any healing. As a dps she's... just terrible.
Given that the game gives you Wednesday and Lionheart before you get Big Sis, there's no reason to ever use Big Sis as your DPS unless you like her aesthetically.
Big Sis' short support skill cooldown does allow her to be used as a support operative though. Whilst she's not brilliant at it, with enough investment she it can work.
By the time you've gotten the investment needed to make her a viable support you will already have better supports, so there is never a reason to use Big Sis on your team unless you just like Enya

The Basics

Role: Off-field Support
Deals Thermal Damage
Deals ballistic and skill damage
Mobility is average

How to use Enya - Big Sis
[as a support]

1. Add her to your team
2. Use another operative as your main dps
3. Spam Big Sis' Support Skill off cooldown

How to build Enya - Big Sis
[as a support]

Weapon: Prismatic Igniter is her ideal weapon
Weapon Parts: Max ATK
Logistics Squad: Twilight
Basic Stats: +19.3 Skill Haste
+10.0% ATK
Special Stat: +8.5% Chaos Damage
Best squadmates: Big Sis can be used as a weak support that works with basically all dps operatives.

Standard Skill
Enya leaps backwards, throwing bombs in front of her in a fan arrangement.
If a target is hit by one bomb, then subsequent bombs that hit that target deal 50% less damage.
This stacks, which means that shotgunning enemies doesn't really result in that much extra damage.

Since Enya throws these bombs upwards, they don't detonate at her feet, meaning hitting nearby enemies is difficult.
This also means that to shotgun a boss, you need to be at point-blank range.

Passive Effect
Enya can have up to 10 stacks of "Rich Light". She gains one stack every 5 seconds.
Each time she hits a target with her gun, she uses 1 stack to shoot a "Snake Feather" projectile that seek an enemy and deals Thermal damage

Ultimate Skill
Enya deals Thermal damage in front of her.
After the bombardment ends, Enya throws 30 "Snake Feather" projectiles that track enemies and deal Thermal damage.

Using her ult also refreshes her Rich Light stacks to max, and increases their generation speed by 100% (i.e .5s -> 2.5s) for 10s.
This increased generation speed means she gains an extra 2 stacks in total as a result of using her ult.

Support Skill
Enya throws 1 sticky bomb to a target in front of her, which explodes after 5 seconds (or 10 seconds with her Neuronic)
With her Neuronic it can also hit another 2 targets.
If you shoot the target before the bomb naturally explodes, it explodes early and deals a bit of extra damage.

This is a weak ability that does not deal much damage, and it is never worth using Big Sis as a support in order to be able to get the damage from this skill.

Note that this skill has a base 15s cooldown, which is quite short.

Weapon

Prismatic Igniter
This 4* weapon gives the deployed operative an ATK buff (based on Enya's ATK) whenever Enya uses her support skill.
The buff lasts 15s, so the uptime is 100% even without any Skill Haste mods.

Since she's pretty terrible as a DPS, Big Sis is best used as a support, using the short cooldown on her support skill to frequently proc support effects, including Prismatic Igniter.

For weapon Parts just go with ATK.

Logistics

Since she's pretty terrible as a DPS, Big Sis is best used as a support, using the short cooldown on her support skill to frequently proc support effects.
With some Skill Haste, Big Sis can proc her support skill quite often, allowing her good uptime on Twilight squad, which gives a 5s 16% resistance debuff to enemies hit by the users support skill.

Big Sis' support skill has a 15s default cooldown.
With 3 Skill Haste mods that can be reduced to 9.5 s.
This gives Big Sis around 53% uptime on Twilight's resistance debuff, which is decent.

For stats you want +19.3 Skill Haste and +10.0% ATK.
For the Special talent, you want +8.5% Thermal Damage.

Manifestation

M1 buffs her ultimate, allowing the Snake Feathers to remain on the target for 3s, dealing Thermal DMG each time the target is shot.
This is actually quite a good damage buff
M2 causes enemies who have a Snake Feather to take 2% more ballistic dmg.
This only works if Enya has a Thermal Weapon, but she should ideally always have one anyway.
Since this is a DMG Taken effect and most of Enya's damage is ballistic, this is very strong.
M3 causes 1 stack of Rich Light to be consumed to release a Snake Feather whenever another operative hits a weakspot, with a 0.5s ICD.
M4 increases the level of her ult by 1, and increases its damage.
M5 causes her ult to throw out 50 Snake Feather projectiles instead of 30.

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

Overview and Good Teammates for Big Sis

Big Sis' Prismatic Igniter weapon and Twilight squad both provide generic buffs that are useful to basically every dps.
As such, whilst she is simultaneously weak, whilst also requiring lots of investment to even become weak, she is universally useful.

By the time you get to the point where you have Prismatic Igniter and Twilight squad, you likely also have a 5* operative, whom you should use and invest into instead, meaning there's no reason to use Big Sis as a support.

Even if you only have 4* operatives, you can make a decent team out of them, but Big Sis doesn't really fit that well with the other 4* operatives, not really being able to make use of many support buffs, whilst not providing much as a support to other operatives, whilst also being a poor dps herself.

Enya is not a strong operative - either as a dps or as a support, and is quite clunky in how she works, so I don't recommend you use her.
If you do get a Prismatic Igniter though, she can work as a support, and tends to be one of the easier operatives to use as a support due to her support skills low base cooldown which means she doesn't need Skill Haste rolls to maintain the Prismatic Igniter buff, although being easy isn't really useful given the investment needed.
Again, by the time you have enough investment to make Enya a competent support you'll have better operatives to work as supports.