Sethos

4★

ElectroBowMain Damage

TL;DR

Sethos stands still and fires fully charged arrows that punch through everything in a line. The clever bit is that his Energy is what makes them charge quickly. Each shot spends it. His Skill hands Energy back whenever it sets off a reaction, so the loop is Skill, then shoot until you are empty. His Burst is a different way to play him that wants the same Energy, so you pick one.

Rotation

Skill (E) into enemies that already have another element on them. The reaction pays him Energy back.
Hold the Charged Attack to full and let go. The arrow pierces everything in the line.
Fire three more of them. His passive bonus covers four shots.
Skill (E) again when it comes back, every 8 seconds, then go round again.

Abilities

Charged Attack	Hold to aim. Let go early for an ordinary Electro arrow. Hold to full for the piercing shot, which goes through every enemy in its path. He cannot move once it is fully charged.
Skill (E)	Electro damage in front of him and a hop backwards, on an 8 second cooldown. If it triggers a reaction he gets Energy back, which is what pays for the next few shots.
Burst (Q)	Costs 60 Energy, 15 second cooldown. For 8 seconds his Normal Attacks turn into piercing bolts that scale off his Elemental Mastery and count as Charged Attack damage. He cannot aim at all during it.
His bonus	A big chunk of Elemental Mastery added to piercing shots. It runs out after four of them land, or 5 seconds after the first one hits, then comes back 15 seconds later.

The one mechanic to understand

Energy is his charge speed.

Every point of Energy Sethos is holding shortens the aim, counting up to 20 points, which brings a full charge down to about a third of a second. Firing the shot spends what it counted, or half of it on an early release. That means Energy is not a resource he saves, it is the thing he spends to shoot quickly. His Skill exists to top it back up off a reaction. It also means his Burst is competing with his own arrows for the same 60 Energy. Fire the arrows and skip the Burst, or hold the Energy for the Burst and accept slow shots in between. Trying to do both leaves you doing neither well.

What actually matters

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

Elemental Mastery. His passive adds 700% of it to every piercing shot, the Burst bolts read off it and his reactions scale with it. It beats Attack on him twice over.
The Skill has to hit an element. Electro-Charged, Quicken, Overloaded, Hyperbloom or an Electro swirl all pay him. On a bare enemy he gets nothing back and his shots crawl.
Four shots per window. The bonus drops after the fourth piercing shot lands and takes 15 seconds to come back, so a fifth shot inside the window is a much smaller hit than the four before it.
He is rooted while charging. Full charge locks him in place, so pick your moment or take the hit.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Elemental Mastery first, Crit second. Slingshot is a 3-star with Crit Rate on it and a bonus that pays out when the arrow hits something close in front of him, which is where his piercing line starts anyway.
Artifacts	4-piece Wanderer's Troupe, since everything he fires counts as Charged Attack damage and the set adds Elemental Mastery on top. It only comes out of the strongbox, so 4-piece Gilded Dreams is the farmable route to most of the same thing.
Main stats	Elemental Mastery sands, Electro damage goblet, Crit circlet. Energy Recharge sands instead if his shots feel slow, since Energy is charge speed here and more of it means more shots.

Works well with

Dendro

Hydro

Electro

Anything that leaves its element sitting on the enemy, since his Skill needs a reaction to pay him and his arrows want one to hit through. A second Electro character covers his Energy when the reactions are thin.

Don't bother

Normal Attacks outside the Burst. They are ordinary bow shots and they do not pierce.
Releasing an aimed shot early. Half charge fires a plain arrow that misses out on the piercing and the passive bonus.
Attack stats. Elemental Mastery does the same job better on him.
Levelling his Skill talent much. It is in the rotation for the Energy rather than the damage, so the Normal Attack talent is where the levels go.

Want the full theorycrafting version? Read Sethos on KeqingMains.