Mia - Starchaser

If you want to try her out, Starchaser is available as a trial operative in story chapter SP7 - "Beyond Elysium" (Main Story // Journey - Allies Revisited // Beyond Elysium)

Starchaser has truly astounding AoE capabilities, very high range, decent damage, and a simple, quick, and easy damage rotation, which comes at the cost of very low mobility, having to aim when shooting single-targets, and potentially spending a lot of her time in combat standing completely still charging up her standard skill attack.
With her M5 Mia really wants an Electric support, which can potentially make teambuilding with Mia tricky.

Mia plays extremely similarly to Bubu - The Geomancer.

The Basics

Role: Main dps
Deals Electric Damage
Weapon Type: Shotgun
Deals entirely ballistic damage
Mobility is dreadful

How to use Mia - Starchaser

Always use Mia's ult the instant the mission starts.

If you can, always reload just before combat starts.

Against single-targets/bosses:

Shoot 3 Hyperchew Shells (the rapid 3-shot burst)
Fire normally to shoot 3 bursts of 2-shots each
Use her ult to fire 8 shots.

Repeat
(This rotation takes around 6s)

Against Groups:

Tap her standard skill and wait for it to fully-charge, then tap again to release and deal AoE damage over a massive area

Repeat
(This averages around 3.0s per shot if spammed)

How to build Ethereal Cloud

Weapon: Skirted Confession / Cherry Blast
Weapon Parts: Max ATK
Logistics Squad:
+10.0% ATK
+10.6% Crit Damage Amplifier
(Alignment Index is a good stat)
+8.5% Electric Damage
Best supports:
Vidya - Celestial Swan
Yao - Nightglow

Specifics:

Standard Attack

Mia has a "Sweetchew Magazine" instead of a standard magazine, which has a fixed ammo capacity of 6 shots,
Shots from this special magazine always crit, and have their base Crit Dmg and Weapon Compatibility both fixed, meaning they are not affected by the actual stats her weapon has

All shotguns have 30% Crit Dmg, and Shotguns tend to have 35-40% Weapon Compatibility, so Mia's increased Crit Dmg and Weapon Compatibility stats mean that she deals around 3x the damage of a regular shotgun and around 4-4.5x the crit damage.

When shooting, Mia will shoot 2 shots at a time at a high rate.
If she only has 1 bullet left in her magazine, she will still shoot 2 shots, but the second shot will deal 30% less Final damage.

If shooting continuously, Mia will automatically reload when her mag empties. Unlike how shotguns normally work, Mia reloads all her shots at once when she reloads this way.
Reloading normally when not shooting still reloads each shell individually as normal, but since she reloads automatically there's no real reason to ever reload manually.
Since she gains buffs after reloading, you are usually better off not reloading after combat, so you reload before the next encounter to gain her reload buffs faster.

Despite being a shotgun user, Mia's shots only deal one instance of damage, instead of shooting 8 pellets that deal separate damage like a regular shotgun.
Mia does not benefit from effects that grant her extra shotgun pellets.

Every other burst Mia will kneel down to fire. Since she is so short whilst kneeling, some enemy projectiles can go clean over her head and not hit her.
This does also have the caveat that where she aims oscillates, as her kneeling does also pull her aim downwards with it.

Note that Mia doesn't have any ADS functionality - she can go ADS while not shooting, but she stops ADS'ing when she actually starts shooting.
This means any bonuses to ADS damage are literally worthless for Mia
Any bonuses to hipfire damage will affect all of Mia's damage.

Mia also can't move whilst shooting, but if you hold down a movement key Mia will move in that direction between shots.
This works identically to how Bubu - The Geomancer works.

This Sweetchew Magazine counts as a special magazine, and thus can't be used alongside other special mags like those from Fenny - Starshine.
Being a special magazine, it can't be refilled by special reloading mechanics - only by manually reloading, reloading automatically when her mag is empty, or by Mia's ult.

Standard Skill

Tap Mia's standard skill to start charging a cone AoE attack. Release by using her standard skill again.
The cone increases in size the longer it is charged, up to a max AoE size/range that is truly vast, and gives Mia truly ridiculous AoE capabilities.
Regardless of charge level, this skill can damage up to 10 targets.
The Damage and Destructible Object DMG also increase the longer it is charged, but it can't crit.

The max charging time is 1.5 seconds.
The edges of the cone visually change from purple to purple+orange when fully charged.

This counts as a ballistic shot, not a skill use/hit.
Despite this, it doesn't deplete her magazine.

Whilst not stated in the skills description, Mia's Standard Skill does technically perform a reload when used.
This doesn't reload her magazine at all, but it does allow her weapon and logistics buffs to work properly even if you just spam her standard skill.
Since her standard skill doesn't deplete her magazine, without this technical reload she would never reload if you just spammed her standard skill, thus preventing effects that trigger upon reloading from working (i.e. her signature weapon/logistics). As such, this is a very useful feature.

This skill does not need line-of-sight to hit enemies - Mia can hit any enemy that is within the AoE, even if they are behind an obstacle.

Shooting also moves/pushes Mia herself backwards a little bit - She is moved back the same amount regardless of how charged the skill is.

Mia can't move while charging, but she can dodge while charging, which doesn't interrupt the charging.
You can use her dodge to move forwards to help keep her in place, as again; her skill moves her back slightly.

When Mia was released, she had no CC resistance whilst charging or shooting, however she does now get some.

Whilst this skill has a 1.5s cooldown, the time Mia takes to actually fire means that if spammed as often as possible Mia will be shooting at a rate of 1 fully-charged shot every 3s.
Since it only costs 2 S-Energy, Mia has no S-Energy issues, as she recovers S-Energy more quickly than she uses it.

Ultimate Skill

Mia reloads and enters "Limit-Break state", which has an unlimited duration. Mia also immediately gains 1 "Hyperglow Mark"
If Mia has a Hyperglow Mark, her next shot consumes it to rapid-fire 3 "Hyperchew Shells" without depleting her magazine.
Each Hyerchew Shell always crits, and has its base Crit Dmg fixed at 100%, and their Weapon Compatibility also fixed.
The Weapon Compatibility value is [a little] different, but otherwise these are the same as her regular shots.
The high rate of fire means the damage from this ult is very high though.

Note that once you enter Limit-Break state, there is no way to exit it in a stage - the only way to exit Limit-Break state is to exit the mission.
Switching to another operative or otherwise removing Mia from the field will not remove the state.

Each shot fired during her ult will give Mia 4 "Sugarcrux".
Each Hyperchew Shell fired during her ult will give Mia 8 Sugarcrux.
Mia can have 48 Sugarcrux max.

When Mia has 24 or more Sugarcrux, she can use her ult again to enter "Sugarnova" stance.
This gives her a "Sugarnova Magazine" equipped with 1 Sugarnova Shell for each 6 Surgarcrux she has (so 8 max), then rapidly auto-fires all of them.
Sugarnova shells always crit, have their base Crit Dmg fixed at 100%, and their Weapon Compatibility also fixed.
When the Sugarnova Magazine is empty, Mia returns to her Sweetchew Magazine as standard.

With its second Neuronic, firing 8 (the max) Sugarnova Shells gives her 1 Hyperglow Mark.
This means you should only ever use Mia's ult when she has maxed her Sugarcrux/ult and can fire the max of 8 shells.

The damage-per-shot isn't necessarily that much higher than her regular shots, but the extreme rate of fire means its dps is very high.
Using Mia's ult at max with its Neuronic also gives a Hyperglow Mark, which increases the dps of her next burst.
Overall, Mia's ult is responsible for a large chunk of her single-target dps.

Note that the AoE shot from her standard skill doesn't give Mia any Sugarcrux, and thus can't be used to charge her ult.

Support Skill

Mia deals PBAoE Electric Damage to up to 5 nearby enemies.

With its first neuronic, it also removes any CCs you are under.

With its second Neuronic, it also knocks targets up into the air.

The damage is very good for a main-dps support skill [not that that says anything], but the long cooldown of 20s means its dps is terrible, and it offers no buffing or debuffing.
Like basically all main dps operatives, Mia is not a good support and you really don't want to ever use her as one.

Note that Mia's standard rotation is 3 Hyperchew Shells, followed by 3 bursts of 2 Sweetchew Shells each, followed by the 8 Sugarnova Shells from her magazine.
This means she shoots 9 non-ult shots and 8 ult shots each rotation.
Combined with the slightly higher Weapon Compatibility the shots from her ult get, this works out to mean that Mia's ult is doing half of all her damage.
(921.6% vs 930%, so almost exactly half)

Weapon

Skirted Confession
Mia's designated 5*, and best weapon.
+18/30% Electric damage is good.
When Mia replenishes her special clip (i.e. she reloads or uses her standard skill) she gains +18/30% ATK.
Each time she crits or uses her standard skill, she gains +18/30% Ballistic DMG for 5s.
When both effects are in effect, increases the Ballistic DMG enemies take by 18/30%.
As usual for modern 5* weapons, increasing the damage enemies take is by-far the strongest part of this weapon.
As always for signature weapons, Mia can maintain all these buffs indefinitely in combat without issue.
Skirted Confession also has an inherent ATK Boost stat of 56%, which is the highest of any shotgun.
Cherry Blast
When Mia replenishes her special clip (i.e. she reloads or uses her standard skill) she increases her ATK by 21-35%.
Whilst that is active, when she shoots she gains 1 stack of +6-10% ballistic damage for 5s, up to 3 stacks.

Remember that Mia's Standard Skill does technically perform a reload when used, so the reloading elements of her signature weapons do still work even if you only spam her standard skill.

Sunny Payback
Sunny Payback has a slightly better reload time than the other electric shotguns Mia wants, and has a higher ATK and %ATK stat than the 4* electric shotguns.
Note that whilst Sunny Payback does have a much higher compatibility stat than most other shotguns (at 41%) Mia fixes Weapon Compatibility, so that isn't useful.
Sunny Payback has a lower magazine capacity than other shotguns, but since Mia fixes ammo capacity that's not a downside for Sunny Payback.
Sunny Payback has a lower rate-of-fire (60 shots/minute) than most other shotguns (which are typically 75 shots/minute).
Sunny Payback's weapon effect is a 60/100% chance for an extra shotgun pellet, but since shotgun pellet count doesn't affect Mia this effect is wasted on her.
Sunny Payback is a decent weapon for Mia, but it isn't really worth using as it doesn't really outperform Cherry Blast as Mia can't really make use of many of its strengths.

Further note that Mia's Deiwos increases her Electric Damage, as does her Alignment Index. [Her M1 buffs electrical damage, and her M3 also requires an Electric weapon.]
As such, even if you have no investment into her at all, Mia is still always best used with an Electric weapon.
Non-Electric weapons will be a notable downgrade in damage due to not benefiting from Mia's various Electric damage buffs.
In an absolute worst-case scenario, Mia loses around a quarter of her damage if not using an Electric weapon.
Even with decent investment, the damage loss from not using an Electric weapon is still quite high.
As such, Mia doesn't really have any cope off-element weapon recommendations - Make sure Mia is using an Electric-damage weapon at all times.
If you don't have any of the above three weapons, then Discordance is her next best weapon.

Since Mia gains buffs after reloading, your dps will increase if you shoot before you enter combat, and have Mia reload immediately as you enter combat so she is freshly reloaded as combat starts, and thus already has her reload buffs, rather than the shots from her first magazine being unbuffed.

For weapon parts Mia wants maximum ATK.

Logistics

Squad

Squad: Nimbus
This is her designated squad, and it is her best choice by far.
Nimbus squad gives Mia +22.5% ATK, Ballistic DMG, and Crit DMG Amplifier for 5s after reloading or using her standard skill.
As stated earlier, Mia can spam her standard skill at a rate of 1 shot every 3s or so, so maintaining Nimbus squads bonus is trivial as long as Mia doesn't get CC'd.

Also note again: she only gains these buffs after a reload, so having Mia reload just before combat starts will proc these buffs, improving the damage of her first rotation.

Note that since Mia can't ADS, the usual BiS non-event ballistic-damage squad of Amano-Iwato is very poor to her - Mia can still use the 2/3 Ballistic damage buff, but the 3/3 ADS buff is useless to Mia.

Since her skills proc ballistic effects, and the skills themselves aren't doing any hits, Thebes is also useless, as is Twilight.

If you don't have Nimbus, other good cope sets for Mia are:

Fenya
+46% DMG for 10s after using her ult.
Since Mia can use her a fully-charged ult at least every 10s, this is basically a permanent buff, and +46% damage is strong.
A decent amount of her damage comes from her regular non-ult shots, but the +32% ultimate damage from the 2-piece is still decent.
Mizuho
+24% ballistic damage is nice.
Mia's low RoF and magazine capacity mean the 3-piece isn't great, but it does give her extra ballistic damage, which is good.
Mizuho isn't a good option, but besides Fenya it's one of the only logistics sets Mia can make full use of.
Mingyi
+24% ballistic damage is nice.
The stacking damage buff after dealing electrical damage is nice, but if Mia gets CC'd she's likely to lose the buff entirely, and it takes a long time to stack up.
Her standard skill takes too long to use for it to work with Mingyi, so against groups Mingyi provides very little benefit.

Stats

Basic Stats: +10.0% ATK
+10.6% Crit DMG Amplifier
Special Talent: +8.5% Electric Damage

As a main dps, Mia obviously wants maximum ATK and Electric Damage to maximize her damage.

Mia's standard skill can't crit, but otherwise all of her damage always crits.
Mia also gains a very high base crit damage stat of 100%.
These combine to mean that Mia gains a lot of damage from Crit DMG Amplifier mods.

Mia gains +5% Electrical damage for each 100 Alignment Index, so a max-roll of +106 Alignment Index gives her +5.3% Electric Damage.
If you only want to spam her standard skill, Alignment Index mods do outperform Crit DMG Amplifier mods since her standard skill can't crit, so Crit mods useless for it.
Mia's damage is plenty for casual content, but for endgame content (e.g. Neural Sim) single-target damage is what's needed, so building Mia for her attacks rather than her standard skill is what's recommended, and hence Crit DMG Amplifier is a much better stat for Mia than Alignment Index.

Since Crit Damage stacks multiplicatively with all other damage factors, but electrical damage mods stacks additively with other damage mods (e.g. ballistic damage mods, or other electrical damage mods) this means that Crit DMG Amplifier mods do outperform Alignment Index mods.

Manifestation

M1 causes Mia's damage to ignore 15% of enemies Electric Resistance.
(It says "When equipped with a special clip", but remember Mia's default magazine is a special clip, so she always has a special clip active, so it's a permanent buff.)
M2 gives +8% Final DMG per 16 Sugarcrux used when equipping the magazine from her ult, up to 24%
With M2, you'll want to make sure Mia is at the max of 48 Sugarcrux before using her ult... which is something you should be doing already anyway.
(You should be doing that anyway becuase her ults 2nd Neuronic buff gives her an extra Hyperglow Mark if she fires her ult while at 48 Sugarcrux, which is good.)
M2 is a very nice damage bonus, but a good chunk of her damage comes from her normal shots, so the overall effect on her damage is less than 24%
M3 causes Mia's shots to ignore 15% of the targets DEF if Mia has an Electric Weapon, which she always should have.
This is a nice damage buff.
M4 increases the level of her ult and increases the Weapon Compatibility (i.e. base damage) of its shots by around 10%.
As always, this is a decent buff, but doesn't change how she plays - it's just more damage.
M5 gives Mia +10% Crit DMG Amplifier if she has an Electric Weapon (i.e. hopefully always) and an extra 10% Crit Dmg Amplifier if the squad has another Electric operative in it.
There are a lot of good electric-damage supports, so the requirement isn't hard to meet, but for newer players who don't have any good electric supports this can be tricky.
Similarly, it can potentially make teambuilding a bit tricky in some situations.

If you want efficiency (good damage without massive investment), then M2 and M3 are a good stopping points. I'd generally recommend M3, but M2 is also not bad.

Good Supports for Mia - Starchaser

Mia deals ballistic damage, has high base crit damage, and wants to maximize her ATK stat.
As such, Yao - Nightglow and Vidya - Celestial Swan are her best supports.

With her M5, Mia really wants at least one Electric-damage support on her team - Yao - Nightglow is the ideal choice for this.

Note that Mia can't use U-Energy, allowing your support operatives to make full use of it.

These supports work well with Mia:

Yao - Nightglow for the ballistic, ATK, and crit buffs. Nightglow is also on-element to Mia.
Vidya - Celestial Swan for the resistance and DEF shredding
If using Celestial Swan, make sure the other support isn't off element to Mia.
Acacia - Kaguya for resistance debuffing if Vidya is busy that week.
Kaguya is off-element to Mia, but since you aren't likely to be using both Kaguya and Celestial Swan to support Mia this isn't much of an issue.
Marian - Aquila
Aquila provides very strong generic damage buffs, which makes her an excellent support.
Aquila is off-element to Starchaser (Aquila constantly deals Kinetic damage).
Eatchel - The Cub's ATK boost is still very nice.
Eatchel doesn't deal damage and thus doesn't interfere with Celestial Swans effects, but Eatchel is technically a Kinetic operative, and thus doesn't proc Mia's M5 buff.
Mauxir - Shadow Ka still works well for the extra damage, as well as providing a stationary target that makes the enemy easier to hit.
Mauxir deals Chaos damage and is off-element to Mia.

Vidya - Agave
Whilst Agave is usually only used as a Parts Breaker, her support skill does give a +20% ATK buff, and she can proc Twilight squad if she has it, which buffs Mia's damage.
Agave is also on-element to Mia, allowing Agave's presence to proc Mia's M5 buff and/or be used with Celestial Swan if you don't have any better supports.
Agave is great parts breaker, but is a bad support, but since Mia really really wants an Electric-teammate, Agave being Electric does elevate her above her usual position.
You are unlikely to have M5 Mia, Agave, and no other decent supports, so you're unlikely to want to use Agave, but if you somehow have nobody else then Agave can work.
Agave is still a weak cope support, but she can work if you don't have any better supports or any other electric support.
Fenny - Coronet
Coronet is an Electric-damage operative, and thus does proc Mia's M5 buff.
Coronet's RoF buff does benefit Mia, although +30% RoF isn't that strong for Mia as most of her time "shooting" is her moving between shots.
Coronet is not a strong support for Mia, but can work if you just need an Electric support to proc Mia's M5.
Chenxing - The Observer
Chenxing is on-element to Mia, and thus can proc her M5 bonus.
As such if you somehow have an M5 Mia and no good 5* supports for her, Chenxing is likely your best bet simply to proc Mia's M5, although Chenxing's healing is nice too.
Mia has a high RoF for a shotgun user, so Tess - Phantasia does work, but Mia's RoF is still overall quite low, so Tess isn't ideal.
Fritia - Little Sunshine is the cope version, but Fritia works ok for a 4* support if you have literally nothing else.
Both Tess and Fritia are off-element to Mia.

These supports don't work with Mia, either well or at all:

Enya - Exuvia doesn't work with Mia, as Mia never deals any skill damage.
Fenny - Starshine doesn't work well with Mia, as Mia's own magazine is a special clip, and thus prevents Fenny's special Clips, meaning all Fenny provides are a few small ballistic buffs, which are not great.
Without the buffs from her Special Clips Starshine performs extremely poorly and isn't recommended at all.