Ayato

5★

HydroSwordMain Damage

TL;DR

Ayato's Skill puts him in a six second state where his Normal Attacks become a blur of Hydro slashes. His Burst drops a field of falling blades that boosts Normal Attack damage for whoever is standing in it. Burst, Skill, swing for six seconds, repeat. That's the whole character.

Rotation

Burst (Q) first. The field lasts 18 seconds and covers everything that follows.
Skill (E). He dashes and leaves a watery double standing where he was.
Normal Attack for the six seconds the state lasts.
Swap out, let your supports work, then come back for the Skill when its 12 seconds are up.

Abilities

Skill (E)	12 second cooldown. He shifts a short distance, leaves a watery double behind him and enters his state for 6 seconds. The double explodes for Hydro damage when enemies come near it or after 6 seconds.
His state	Normal Attacks become fast Hydro strikes that nothing can override and he's harder to knock around. Charged Attacks and plunges are off the table. Every strike that lands stacks a damage bonus read off his Max HP, up to four stacks. Swapping away clears the state and the stacks.
Burst (Q)	80 Energy, back in 20 seconds. Blades rain on an area for 18 seconds, doing Hydro damage and boosting the Normal Attack damage of anyone fighting inside it.

The one mechanic to understand

Six seconds on, twelve seconds of cooldown.

The state is half as long as its cooldown, so Ayato only gets the field for half the fight and the other half belongs to your supports. Pressing the Skill starts him at two stacks and the double exploding takes him straight to the maximum, so his opening swings hit near full strength rather than ramping up. That's worth knowing because it means short windows lose you nothing.

What actually matters

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

Attack and Crit first, Max HP after. The stacking bonus reads off his Max HP, so HP substats are genuinely worth something, but they don't beat the usual damage stats in the main slots.
Level his Normal Attacks first. The Skill talent only changes what those attacks do. Almost everything he deals is Normal Attack damage.
Energy Recharge, though less than the 80 cost suggests. While he's off the field with under 40 Energy he regenerates 2 a second, which fills most of the bar during your supports' turn.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Crit and Attack. Amenoma Kageuchi is craftable in Inazuma. It banks a charge each time he presses the Skill and cashes them in for Energy when he Bursts, which suits an 80 cost Burst.
Artifacts	4-piece Heart of Depth. Hydro damage on the two-piece, then 30% Normal Attack damage for 15 seconds after the Skill, which is exactly the window he's swinging in.
Main stats	Attack sands, Hydro damage goblet, Crit circlet. HP is fine in the substats but not worth a main slot.

Works well with

Electro

Cryo

He applies Hydro constantly while he's swinging, so anything that reacts with it gets set off over and over. Electro and Cryo are the two that hold up under that rate of application.

Don't bother

Charged Attacks. The state takes them away. Outside it he shouldn't be on the field anyway.
Counting his stacks. The Skill and the exploding double hand him the maximum by themselves.
Full HP builds. The bonus is real but small next to Attack and Crit.
Parking a caster in his Burst field and expecting a buff. It only boosts Normal Attack damage.

Want the full theorycrafting version? Read Ayato on KeqingMains.