Chevreuse

4★

PyroPolearmSupport

TL;DR

Every Overloaded reaction your team sets off loads a heavy round into Chevreuse's musket. Hold her Skill to fire it and the party gets a big attack bonus for 30 seconds, plus a heal that runs while you play someone else. Her Burst is a grenade, and it is mostly there to start the reaction that loads the round. The catch is enormous: her party has to be Pyro and Electro characters and nothing else.

Rotation

Burst (Q) first. Its Pyro sets off an Overloaded on an Electro-soaked enemy, and that loads her round.
Hold Skill (E) and fire it. This is the attack buff, and it starts the healing.
Swap to your damage dealer and keep setting off Overloaded to hold the resistance shred up.
Back in every 20 seconds or so for the same two buttons.

Abilities

Skill (E)	Tapped, she shoulders the musket and fires a quick shot for Pyro damage. Either version also starts a heal that tops up whoever is on the field for about 12 seconds, off her Max HP. 15 second cooldown.
Holding the Skill	Takes aim for a precise shot. If she has a loaded round it goes off instead, for more damage over a wider area, and firing it is what hands out the attack bonus.
Her loaded round	She gets one every time somebody in the party triggers an Overloaded, and she can only hold one at a time. No round means the held Skill fires an ordinary shot and buffs nobody.
Burst (Q)	Costs 60 Energy, 15 second cooldown. Lobs a grenade that splits into a ring of smaller shells, which go off a moment later for more Pyro damage.

The one mechanic to understand

Pyro and Electro only, no exceptions.

Her best trick is knocking 40% Pyro and Electro resistance off anything hit by an Overloaded, and the passive behind it checks your whole party before it will do anything. Every character has to be Pyro or Electro, and there has to be at least one of each. Slot in a single Anemo or Hydro teammate and the shred switches off entirely. There is no partial credit and no way around it, which is why she is either the character a team is built around or a character you leave on the bench.

What actually matters

The two or three real decisions. Everything else is noise.

Max HP, up to about 40,000. The attack bonus is 1% per 1,000 of her Max HP and it stops climbing at 40%, so 40,000 is the target and anything past it does nothing. Her healing reads off the same number.
Firing a loaded round every rotation. An Overloaded has to happen before you hold the Skill, or there is nothing loaded and the whole attack bonus quietly does not happen. Get the order wrong once and you will not notice, which is what makes it worth watching.
Not much Energy Recharge. Both her buffs come off the Skill, so nothing important breaks when the Burst is late. Around 130% and casting it whenever it is up is fine.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Max HP, then Energy Recharge. Black Tassel is a 3-star with a big pile of HP on it and gets her most of the way to the cap for free. Favonius Lance instead if your Pyro and Electro characters are short on Energy, since nobody else in that team is likely to be feeding them.
Artifacts	4-piece Noblesse Oblige. Her Burst goes off every rotation anyway and the party attack bonus it gives stacks on top of her own.
Main stats	HP sands, HP goblet, HP circlet, with a Healing Bonus circlet instead if she is the only thing keeping your team alive. Ignore crit completely.

Works well with

Pyro

Electro

These two are a requirement rather than a preference. What she really wants is a teammate applying one of them off the field constantly, so Overloaded keeps happening while your damage dealer is busy.

Don't bother

Her normal attacks. There is a four-hit spear string in there and she has no reason to ever use it.
Building her for damage. Crit stats on Chevreuse are wasted.
Pushing HP past the cap. Once the attack bonus maxes out, extra HP only buys slightly bigger heals.
Levelling her talents past what you need for the healing. The attack bonus and the resistance shred are flat numbers that do not care about talent level at all.

Want the full theorycrafting version? Read Chevreuse on KeqingMains.