Varka

5★

AnemoClaymoreMain Damage

TL;DR

Varka's Skill puts him into a two-claymore mode for about 12 seconds where he swings hard and his second blade borrows an element off your team. Inside that mode the Skill button becomes a heavy special strike with two charges, and his charged attack spends one of those charges for free. The catch is who you bring: he wants one other Anemo character plus two characters sharing an element, and getting that wrong costs him more than any artifact ever will.

Rotation

Supports first, and let one of them swirl something so he picks up his stacks.
Tap Skill (E) to enter the mode.
Normal attack strings, each one finished with a charged attack to spend a charge.
Press Skill (E) again for the other charge, then get out around the 12 second mark.

Abilities

Skill (E)	Tap to enter the two-claymore mode for about 12 seconds. Holding it is a charging leap for getting up hills, on its own short cooldown. The tap version is about 16 seconds.
Skill (E) in the mode	The special strike, two charges on about 11 seconds each. Hitting things with normal attacks knocks time off that cooldown.
Charged attack in the mode	Spends one charge of the special strike and costs no stamina. Same idea, different button, so use it to empty the charges before the mode runs out.
Burst (Q)	Costs 60 Energy. Two big slashes and that's it. It's the least important thing he does.

The one mechanic to understand

His team composition is a damage multiplier.

Two conditions, and they stack. One: at least two Anemo characters in the party, so him plus one more. Two: at least two characters sharing any one of Pyro, Hydro, Electro or Cryo. Hitting one of those puts his attacks at 140% of what they were. Hitting both puts them at 220%. That is not a bonus, that is the character, and a team of four unrelated elements plays like a completely different unit. His second blade also picks an element off the party by priority, Pyro first, then Hydro, Electro and Cryo, so the pair you bring is the element you get.

What actually matters

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

Crit, then attack. Nothing clever, he's a front-liner who wants the usual stats.
The Skill talent runs his normal attacks too. Everything he swings in the mode reads off the Skill talent level, so that's where the books go and the normal attack talent can stay where it is.
Skip Energy Recharge. His Burst is a small slice of his damage and his passives don't touch it. Leave the talent at level one and spend the stats on crit.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Crit first, attack second. Blackcliff Slasher is in Paimon's shop for starglitter if nothing in your chest has crit on it.
Artifacts	4-piece A Day Carved From Rising Winds. 4-piece Shimenawa's Reminiscence is the older stand-in and costs him almost nothing, since he wasn't casting the Burst anyway.
Main stats	Attack sands, attack goblet, crit circlet. An elemental damage goblet can beat attack, but only if it matches the element his second blade has borrowed, so attack is the one that's always right.

Works well with

Anemo

Pyro

Hydro

Electro

Cryo

One more Anemo character, then two characters sharing any single one of the other four. Pyro gets picked for his blade first if you've given him a choice.

Don't bother

Plunging attacks. He can't do them in the mode, which is where he lives.
Holding the Skill in a fight. It's travel, not damage.
Mixing elements for coverage. Two Pyro teammates beat one Pyro and one Electro every single time, because only a matching pair counts.
Setting up swirls for him on purpose. His stacks come from teammates swirling, which happens by itself in a team built the way he wants it.

Want the full theorycrafting version? Read Varka on KeqingMains.