Lauma

5★

DendroCatalystSupport

TL;DR

Lauma is here to make somebody else's Bloom damage much bigger. Her Skill plants a grove that strips Dendro and Hydro resistance off enemies. Her Burst hands out a pile of stacks. Each stack gets spent by one instance of the party's Bloom damage to make it hit harder off her Elemental Mastery. Having her in the party also turns everyone's Bloom into the moon version of it. Two buttons, then swap off and let the team react.

Rotation

Your Hydro teammate goes first so there is water on the enemies for your Dendro to bloom.
Skill (E). Hold it if the dew counter has anything in it, tap it if not.
Burst (Q) straight after, which converts what the hold gave her into more stacks.
Swap off for the next 15 seconds while the team triggers the reactions that spend them.

Abilities

Skill (E), tapped	Dendro damage around her and plants the grove, which sits there hitting things for 15 seconds. Anything she or the grove hits loses Dendro and Hydro resistance for 10 seconds. 12 second cooldown.
Skill (E), held	Needs at least one dew. Spends up to three of them for a second hit counted as Lunar-Bloom, on top of everything the tap does. Each dew spent leaves her a moon song stack for 15 seconds.
Burst (Q)	Costs 60 Energy, 15 second cooldown. Gives her 18 stacks. Any moon song she is holding, or picks up in the 15 seconds after, is converted at six stacks each.
Her stacks	Bloom, Hyperbloom, Burgeon and Lunar-Bloom damage from anyone in the party spends one stack and comes out larger, by an amount read off her Elemental Mastery. Each stack lasts 15 seconds from when she got it.
Charged Attack	Turns her into a floating form that charges forward for up to 10 seconds with two jumps in it, on stamina. A travel move.

The one mechanic to understand

The stacks are the buff and reactions eat them.

The Burst alone gives 18 stacks. Hold the Skill first with three dew banked and those become three moon song stacks, which the Burst turns into 18 more. Every instance of Bloom damage in the party spends one stack, so hitting five enemies at once spends five. That is not a problem, it is where the damage went, but it does mean the stacks disappear fast in a crowd. The dew itself comes from the party triggering Lunar-Bloom, one dew at most every 2.5 seconds up to a cap of three, so her first rotation of a fight is always a tap.

What actually matters

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

Elemental Mastery. Every number she hands the party reads off it: what each stack adds, up to 14% more base damage on Lunar-Bloom, up to 32% on her own Skill. Nothing else on her comes close.
A Hydro teammate. Bloom is Dendro landing on water. She supplies one half of that and nothing in her kit supplies the other.
A second moon character. On her own the party sits at the lower Moonsign, where her Skill lets everyone's Bloom, Hyperbloom and Burgeon crit at a fixed 15% Crit Rate and 100% Crit DMG for 20 seconds. With a second one the level goes up and the buff becomes 10% Crit Rate and 20% Crit DMG on Lunar-Bloom instead.
Energy Recharge. The Burst is most of the character and it costs 60. Around 180% if she is your only Dendro, quite a bit less with a second one feeding her particles.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Elemental Mastery, as much of it as you can find. Mappa Mare is craftable and carries it. Etherlight Spindlelute instead if the Burst keeps arriving late: it carries Energy Recharge and grants Elemental Mastery of its own after her Skill lands.
Artifacts	4-piece Silken Moon's Serenade, which gives the party Elemental Mastery and boosts the moon reactions she is converting everything into anyway. 4-piece Instructor covers the first half for free while you farm.
Main stats	Elemental Mastery sands, Elemental Mastery goblet, Elemental Mastery circlet. Energy Recharge sands instead if the Burst is late. Ignore Crit, since the damage her stacks boost belongs to whoever triggered the reaction.

Works well with

Hydro

Electro

Pyro

Hydro is the requirement. Electro or Pyro turn the blooms into Hyperbloom or Burgeon, which her stacks make bigger just the same.

Don't bother

Her Normal Attacks. She is on the field for two buttons.
Crit stats on her artifacts.
Tapping the Skill when you have dew banked. The hold does everything the tap does with an extra hit on top.
Using the floating form as an attack. It is how you cross the map, not part of a rotation.

Want the full theorycrafting version? Read Lauma on KeqingMains.