Nefer

5★

DendroCatalystMain Damage

TL;DR

Nefer's Skill puts her in a dance state where her Charged Attack becomes a free dash that costs no stamina and spends a dew instead. That Charged Attack is the whole character: it hits, it counts as a moon reaction and it hoovers up the seeds your team's blooms left on the ground. She needs a Hydro teammate to make those blooms happen at all.

Rotation

Your Hydro teammate goes first so there is water on the enemies for your Dendro to bloom.
Skill (E) to charge in and start the dance state, which lasts 9 seconds.
Charged Attack three times. Each one spends a dew and picks up any seeds near her when it lands.
Skill (E) again on the second charge and repeat. Fit Burst (Q) in whenever you have three veils banked.

Abilities

Skill (E)	Charges forward for Dendro damage and starts the dance state, 9 seconds long on a 9 second cooldown with two charges. It also converts every Dendro core already on the field into a seed. On top of that, the party's moon blooms leave seeds instead of cores for the next 15 seconds.
Charged Attack	Ordinarily a stamina-hungry slide. In the dance state, with at least one dew banked, it becomes the free version: no stamina, spends one dew and gathers up the seeds within range of where it lands. Three of them fit in one dance state.
Burst (Q)	Costs 60 Energy, 15 second cooldown. One big Dendro hit in front of her. It spends all her banked veils to hit harder.
Her veils	One per seed picked up. At three of them she gets 100 Elemental Mastery for 8 seconds. They are also what the Burst cashes in.

The one mechanic to understand

Two resources and neither one is hers.

The dew is a party bar that fills when anyone triggers Lunar-Bloom, at most one every 2.5 seconds up to three. Each free Charged Attack costs one, so if the team is not reacting she runs dry and her damage stops. The seeds are what your blooms turn into while she is dancing, in place of the usual Dendro cores. They do not explode and they cannot Hyperbloom, so nobody else gets anything out of them. She picks them up by Charged Attacking near them. Each one is a veil towards her Elemental Mastery bonus and her Burst.

What actually matters

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

Elemental Mastery. Her damage is moon reaction damage, so Elemental Mastery moves all of it. The passive that speeds up her dew only starts paying above 500 of it.
A Hydro teammate. No water on the enemy means no blooms, which means no dew and no seeds. This is the requirement, not a preference.
A second moon character. Below the higher Moonsign level she gets no seeds at all, which takes out her veils, her Elemental Mastery bonus and most of her Burst.
Energy Recharge. Around 100% is enough. The Burst is a bonus on top of the Charged Attacks rather than the thing you build around.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Elemental Mastery first, Crit second. Mappa Mare is craftable and gets her going.
Artifacts	4-piece Night of the Sky's Unveiling, the set written alongside the moon reactions. 4-piece Gilded Dreams is the farmable stand-in, since it is a straight pile of Elemental Mastery.
Main stats	Elemental Mastery sands, Elemental Mastery goblet, Crit circlet. A Dendro damage goblet is worse here, because reaction damage ignores it.

Works well with

Hydro

Dendro

Hydro is the requirement. A second Dendro character keeps blooms coming while she dances. One that carries the moon mark covers the Moonsign level at the same time.

Don't bother

Charged Attacks outside the dance state. They eat 50 stamina and none of the good half applies.
Her Normal Attacks. Three Charged Attacks per dance state is what you are there for.
Bringing an Electro character for Hyperbloom. Her seeds cannot trigger it, so the two mechanics cancel each other out.
Holding the Burst for a bigger cash-in. Three veils is the cap you will normally have, so use it and get back to dancing.

Want the full theorycrafting version? Read Nefer on KeqingMains.