Aino

4★

HydroClaymoreSupport

TL;DR

Aino's Burst drops a little water-spraying duck that sits there pelting enemies with Hydro for about 14 seconds while you go and play somebody else. Her Skill hooks her forward into the fight and does a bit of damage on the way. That is the character: two buttons, then leave, and somebody else reacts with all that water. She raises the party's Moonsign on top, and she is desperately hungry for Energy Recharge.

Rotation

Skill (E) to close the gap and put a little Hydro out.
Burst (Q) to drop the duck.
Swap off and spend the next 14 seconds on the rest of your team.
Back in every rotation for the same two buttons.

Abilities

Skill (E)	Throws a hook forward and drags her along behind it, hitting for Hydro on the way and again where she stops. Hold to pick the direction. 10 second cooldown. Mostly a travel move that happens to do damage.
Burst (Q)	Costs 50 Energy, 13.5 second cooldown. Plants the duck, which stays for about 14 seconds and fires water balls at whatever wanders near it while she is off the field. This is the whole reason she is in your team.

The one mechanic to understand

Half of what she brings is the Moonsign level.

Aino carries the moon mark, so having her in the party pushes its Moonsign up by one. On her own that leaves the party on the lower tier, and with one more moon character it goes up a level. That upgrade makes her own duck fire faster over a wider area, and it switches on the better half of several of the newer characters at the same time. So she often ends up in a team as much for the level as for the water, and pairing her with another moon character is worth more than any single stat on her artifacts.

What actually matters

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

Energy Recharge, far more than feels reasonable. The Burst is the character and she is bad at paying for it herself, so around 200% is the honest number if nothing else in the team is feeding her. It drops a long way with a Favonius weapon or a second Hydro character.
Elemental Mastery. Her duck adds half her Elemental Mastery to its own damage, and every reaction her Hydro sets up for the team scales off it too. It is the only offensive stat on her worth the slot.
Somebody to react with the water. She applies Hydro and nothing else. Electro or Dendro in the team turns that into real damage, and without either she is a duck making splashing noises.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Energy Recharge first, Elemental Mastery second. Favonius Greatsword fixes the exact problem she has, though it needs enough Crit Rate to actually land the crits its passive wants. Master Key is craftable and carries both stats without that condition.
Artifacts	4-piece Silken Moon's Serenade, which hands the party Elemental Mastery and stays up the whole time on her. 4-piece Instructor does much the same job for free while you farm it.
Main stats	Energy Recharge sands, Elemental Mastery goblet, Elemental Mastery circlet. A Hydro damage goblet works but loses, since most of what she is worth is reaction damage other people are triggering. Ignore crit unless you gave her a Favonius weapon.

Works well with

Electro

Dendro

Electro gives you the moon version of Electro-Charged, Dendro gives you Bloom, and either way the reaction is the payoff rather than her own numbers.

Don't bother

Her normal attacks, and the spinning charged attack that eats 40 stamina a second. She is off the field.
Building her for damage. Crit stats do almost nothing here.
Levelling her talents much. Her own numbers are small either way, and Elemental Mastery moves them more than a talent level does.
Using the Skill as an attack. It is how she gets to the enemies so the duck lands in the right place.

Want the full theorycrafting version? Read Aino on KeqingMains.