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. She's not brilliant at it, but it does work.
By the time you've gotten the investment needed to make her a viable support, you will already have better supports, meaning there is never a real reason to use Big Sis.

The Basics

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

Advantages

Support skill is spammable due to its short cooldown
Spammable support skill allows her to proc several decent support buffs quite often.

Disadvantages

Terrible as a main DPS
Not actually great as a support
Needs a lot of investment to become even a mediocre support

How to use Enya - Big Sis

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

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 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 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 generatino 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 (o 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 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.
Twilight squad 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

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

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 instead.