If you don't have her yet but want to try her out, Coronet is available as a Trial Operative in Story Chapter 7 "Into the Mud" (Main Story // Legacy // Containment Zone - Internal Strife)
Powercreep has meant that Coronet isn't a powerhouse of dps anymore, but she still provides good damage with strong parts break in a fun package, plus decent utility as a support operative as well.
Role:
Main DPS w/ good Parts Break
or
Support: RoF buffs + Parts Break
Deals Electric Damage
Weapon Type: Shotgun
Deals ballistic damage
Mobility is decent
Spam her ultimate skill off-cooldown
Spam her standard skill off-cooldown
Otherwise, just shoot stuff
Spam her support skill off-cooldown
If Parts Break is needed, swap Coronet in, Parts Break, and then swap her back after.
Weapon: Sunny Payback / Discordance
Weapon Parts: Max ATK
Logistics Squad: Amano-Iwato
Basic: +10.0% ATK
+10.6% Crit Damage Amplifier
Special: +8.5% Electric Damage
Best supports:
Vidya - Celestial Swan
Yao - Nightglow
Standard Attack and Passive Effect
Fenny's Deiwos gives her a 25% chance of an extra shotgun pellet.
Her ideal weapon is Sunny Payback, which can also give extra shotgun pellets.
These mean that Coronet tends to fire more shotgun pellets than other shotgun users, and thus she benefits more from per-shot effects, as those trigger for each pellet.
(For example the aptitude buffs of Tess - Phantasia and Fritia - Little Sunshine.)
The extra pellets also makes her a bit better at breaking destructible parts than other shotgun users
Standard Skill
Her Standard Skill gives her 80 Mercy points.
Each point of Mercy increases her Rate-of-Fire by 1%.
Mercy slowly ticks down over time, but each shot that Coronet hits gives her an extra 1.2 Mercy.
This skill lasts 12 seconds, or until Coronet runs out of Mercy, whichever comes first.
Whilst in this state, Coronet deals less damage to destructible parts and gains less U-Energy.
...although given her higher RoF, she doesn't necessarily end up with less parts damage or U-Energy.
Ultimate Skill
Coronet deals PBAoE Electrical damage every 2 seconds.
This skill deals decent AoE damage to groups, but it's still a pretty weak skill and doesn't buff Coronet's shotgun, which is where all of her other damage comes from.
If you have a decent support operative to support Coronet with, then you're almost certainly better off using your U-Energy for them rather than Coronet.
Support Skill
Coronet's Support Skill increases the RoF of the deployed operative by 30% for 8 seconds, or 10 seconds with her Neuronic.
Given that it has a 20s cooldown, the base uptime of this skill is 50%, but with some Skill Haste mods on her Logistics she can get much higher uptime.
Coronet wants an electrical shotgun. Good weapon choices for her are:
Sunny Payback
A strong Electrical Shotgun, and Coronets best weapon.
Extra pellets = Extra damage.
Discordance
A good Electrical Shotgun that gives extra Electrical DMG.
Sunny Payback is Coronet's best weapon.
If you don't have that you should use Discordance.
Ambergris
Weaker than Discordance, but since it's an electrical shotgun it's still a good choice for Coronet should you not have Sunny Payback or Discordance
If you don't have any of those, just use Malbec - the 3* electrical shotgun she comes with by-default.
Alternatively, use any high-level shotgun until you can get a 4* or 5* electrical shotgun.
For Weapon Parts Coronet wants max ATK.
For stats Fenny wants +10.0% ATK and +10.6% Crit Damage.
For the Special talent, Fenny wants +8.5% Electrical Damage
Coronet has both a low base crit damage stat (30%) and a high crit chance (25% from her shotgun, 28% from her Deiwos, and 12% from her Alignment Index = 65%)
(Remember that shots against enemy weakspots have a 100% crit chance, meaning any extra crit rate mods she has are literally worthless when shooting a weakspot.)
This means that Crit DMG Amplifier mods work out to be her strongest damage increase, although when not shooting at a weakspot Crit Rate and Alignment Index mods do work out only slightly worse:
119.5% Damage factoring in crits, but with no extra Alignment Index, Crit Rate, or Crit DMG Amplifier mods
123.32% Damage with 3 max-roll Alignment Index mods
124.27% Damage with 3 max-roll Crit Rate mods
125.7% Damage with 3 max-roll Crit DMG Amplifier mods
Again though; Alignment and Crit Rate mods are useless if shooting at a weakspot, but Crit DMG Amplifier mods are her best non-ATK stat regardless of what she's shooting at.
This means that whilst a Crit DMG Amplifier mod is Coronet's best stat after ATK, Crit Rate and Alignment Index are both very good cope stats that perform very similarly when not shooting at a weakspot.
Note that if using Yao - Nightglow to support Coronet, then Crit DMG Amplifier mods are ever stronger still, due to Yao's strong crit dmg buffs and Coronets extremely high crit rate.
Being one of the OG 5* operatives, Coronet doesn't have her own designated logistics squad, so she has a few options, with two optimal squads depending on how she is played:
Amano-Iwato
Increases ADS Ballistic DMG by 10% after a crit. Max 5 Stacks. A great choice if you are ADS'ing with Coronet due to her very high crit chance.
If you only hipfire and if you don't ADS then it's obviously terrible, but Coronet's high crit rate gives Amano-Iwato great synergy with her, so ADS'ing with Amano-Iwato is the best way of using Coronet.
Mingyi
When dealing Electrical DMG, increases ATK of the equipping operative by 1% for 3 sec. Max 45 stacks. Repeated triggers will reset the duration.
Mingyi is still less total damage increase than Amano-Iwato, but Mingyi doesn't need ADS, although does require Coronet fire basically continuously.
Given that Coronet's ultimate skill is terrible and her Standard Skill doesn't need to be used often (literally once per fight with her first Manifestation), shooting continuously is basically the only thing that Coronet does in combat, so sustaining Mingyi is actually quite easy in practice.
Coronet's 5* weapon (Sunny Payback) has a 2.62s Reload SPD, so you'll need to shoot immediately after reloading, but provided you do that sustaining Mingyi is easy.
TL:DR: Use Amano-Iwato if you ADS, and Mingyi if not.
M1 removes the timer from her standard skill, and it now just expires when she runs out of Mercy.
M1 allows Fenny to sustain her Mercy indefinitely provided she keeps shooting, although she can't generally keep her Mercy maxed with just M1.
This takes Fenny from 60% uptime to potentially 100% uptime, which is a massive increase.
With M1 Fenny also doesn't need to stop shooting to re-cast her standard skill.
M1 does thus result in both a nice QoL buff for Fenny, but also a large dps increase as well.
M2 increases the amount of Mercy each shot grants when under her standard skill from 1.2 to 1.7
This only works if she has an electrical weapon, but if you've gotten her to M2 then you should have already gotten her one.
This is quite a large increase, and thus allows her standard skill to be stacked much higher.
M2 allows Fenny to easily stay at max [80] Mercy without too much issue.
M3 removes the Mercy cap (by-default 80) allowing Fenny to stack Mercy even higher than that.
Having higher Mercy does give her a longer grace period and allow her longer downtime before it runs out.
This is thus both a nice QoL change as well as technically being an uptime buff for her standard skill.
The higher Fenny's Mercy, the faster her Mercy decays, so M3 doesn't allow her to stack infinite Mercy.
How much of a buff M3 is depends on what type of content you're doing and thus how high you can normally stack her Mercy, but it's potentially a good buff.
M4 gives Fenny's standard skill +1 level, increasing the RoF bonus Mercy gives from 1% to 1.05% per Mercy.
A 5% increase to her RoF buff is a small increase, but the +1 level to her standard skill is nice. Overall it's not that strong for an M4 bonus compared to modern operatives.
M5 gives Fenny +15% Electrical damage whenever she has 80 or more Mercy.
This will happen a lot of the time in combat, and thus this is a nice damage increase, although sadly it's not a Final DMG mod and is just a regular Electrical DMG mod.
Given that this is just +15% Electrical Damage, this is extremely weak for the investment needed to get it, but it's not bad overall.
Since Coronet is available as part of the standard gacha banner, you will get her eventually (and eventually max her) even if you don't explicitly try to pull for her.
If you did want to actively pull for her, then if you want want efficiency (good damage without massive investment), then M2 is a good stopping point.
If you can stack Mercy or keep it at or near-max, then M3 is instead an efficient stopping point.
Coronet wants ATK, Electrical, Ballistic, and Crit buffs:
Vidya - Celestial Swan (and other resistance shredders)
Celestial Swan and other resistance shredders (Acacia - Kaguya, etc.) that reduce the enemies resistance help enemies take more damage.
If using Celestial Swan, make sure the other support isn't off-element to Coronet.
Yao - Nightglow
Coronet can make full use of Nightglow's ATK, Ballistic, and crit buffs, and Nightglow is a fantastic support for Coronet.
Note that Nightglow is on-element to Coronet (Nightglow deals Electrical damage)
Coronets high crit rate means that Nightglow is generally Coronets one Coronet's two best supports as coronet gets a lot of use from Yao's Crit Dmg debuff.
This is especially true if Coronet has Crit DMG Amplifier mods on her logistics.
Celestial Swan and Nightglow are Coronet's best supports.
Eatchel - The Cub
A Blitzing Fangs M1 Eatchel provides healing as well as a decent ATK buff.
Not the strongest support, but still good.
Mauxir - Shadow Ka
Shadow Ka is quite nice against highly mobile targets (e.g. Esther) but even when her utility isn't needed the extra damage is still very nice.
Note that Shadow Ka is off-element to Coronet (Shadow Ka deals Chaos damage)
Tess - Phantasia
Extra damage per pellet adds a lot for Coronet due to her shotgun shooting 8 pellets, 1 more from Coronet's passive, and Sunny Payback having an extra one too.
Tess adds Frost damage, meaning Tess can't be used with Celestial Swan.
Fritia - Little Sunshine
The cope version of Phantasia.
Little Sunshine adds Thermal dmg per shot, although much less than Phantasia.
Coronets best supports are Nightglow and Celestial Swan.