Nilou

5★

HydroSwordSupport

TL;DR

Nilou only works in a party of nothing but Hydro and Dendro characters. Get that right and finishing her Skill dance upgrades every Bloom seed your team makes into a bigger one that pops almost immediately, which is where all the damage comes from. Her Burst is a lump of Hydro on the way past. She does very little herself, so you dance, swap off, and let your Dendro character set things off. Stack Max HP, because the upgraded seeds hit harder the more of it she has.

Rotation

Burst (Q) to soak everything in Hydro.
Skill (E) to start the dance, then press it three more times to finish it.
Swap to your Dendro character and set off blooms for the next 30 seconds.
Heal the team back up at some point, because those blooms hurt you too.

Abilities

Skill (E)	The first press starts her dancing for 10 seconds. Three more presses are the three dance steps, and the third one finishes on a water wheel that leaves an aura trailing your active character, soaking enemies for about 12 seconds. Finishing the dance is what hands the party the buff. 18 second cooldown.
Burst (Q)	Costs 70 Energy, back in 18 seconds. A lotus of Hydro damage that scales off her Max HP, plus a delayed second hit on everything it caught. It is the only real damage she does with her own hands.
Normal attacks	Pressing normal attack three times while she is dancing is the other way to finish it, ending on a projectile and leaving her swinging Hydro for 8 seconds. Same buff either way, but it keeps her on the field and makes her the one triggering the blooms, and therefore the one taking the damage. The four skill presses are simpler.

The one mechanic to understand

The party list is the mechanic.

Every character in the party has to be Hydro or Dendro, with at least one of each alongside her. Slot in a single Electro or Pyro character and the buff does not apply at all, and she goes back to being a Hydro character with unremarkable numbers. Your healer counts, so it has to be a Hydro or Dendro one. When it is met, Blooms make bigger seeds that pop faster, cover more ground, and hit harder the more Max HP she has.

What actually matters

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

Max HP on everything. Past 30,000 HP, every thousand more makes the upgraded seeds hit harder, and the ceiling is high enough that you will not get near it. HP sands, HP goblet, HP circlet, HP substats.
A healer, and a Hydro or Dendro one. Bloom seeds damage your own party when they pop, and hers pop faster and cover more ground. A bloom team with no healing chips itself to death.
Energy Recharge. Her burst costs 70 and she does not generate much on her own, so either push Energy Recharge up towards 160% or accept casting it every second rotation. Skipping it costs less than it would on most characters, since the buff comes off the skill.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge first, Max HP second. Fleuve Cendre Ferryman is free if you have done any fishing in Fontaine and its Energy Recharge is the stat she is short of. Skip Favonius Sword here: it wants crit hits to make particles and she has no crit.
Artifacts	Two HP sets glued together, because no set boosts bloom damage. 2-piece Tenacity of the Millelith with 2-piece Vourukasha's Glow is the usual pairing, and any other two HP sets do the same job.
Main stats	HP sands, HP goblet, HP circlet. Crit is worth nothing on her.

Works well with

Hydro

Dendro

On this page that list is a hard requirement rather than a suggestion. Anything else in the party switches her off.

Don't bother

Crit stats. Her own damage is a rounding error next to the seeds.
Levelling her normal attack talent. Her talent levels do not touch the bloom buff at all.
Elemental Mastery on her, unless she is the one triggering blooms. It belongs on whoever is.
Hanging around on the field after the dance. The buff is out, the aura follows whoever is active, so go and use it.

Want the full theorycrafting version? Read Nilou on KeqingMains.