Yoimiya, full name Naganohara Yoimiya (長野原宵宮), is the owner of Inazuma's largest and most famous fireworks store.
Talent priority is AA >= E > Q. As Fontaine introduced characters that gain +3 levels on NA via constellations, Yoimiya would benefit from constellations that level her AA and E.
Itto would probably benefit from AA + Q. Alhaitham AA + E. Scaramouche AA + E/Q. Ayaka AA + Q. Eula AA + Q. Diluc AA + E. Xiao is interesting as it depends on constellation: for < C6 then AA + Q, but for C6 would want E + AA/Q.
Genshin's treatment of Yoimiya is a good indicator of how civilized the staff are currently.
Yoimiya has the highest Normal Attack scalings among all Bow users. Yoimiya is a ranged unit that is effective when there are many enemies spread far apart and far away from each other (e.g. Floor 9 of the Version 2.0-2.3 abyss or the Version 2.3 dungeon event). She is useful in hunting down Specters. Because she auto-attacks, people do not have to worry as much about aiming and missing, as is usually the problem with Ganyu.
Her charged shot summons 3 kindling arrows that can home in on targets. These are more for decoration and party tricks as the bulk of her damage is from auto-attacks.
This is multiplicative with her normal attacks. It is one of the few multiplicative buffs in the game. Casting it also deals poise damage, which can pop Mona's bubble and break Electro crystals.
With one of her passives, she gains more Pyro DMG Bonus over time. She gains a stack on hit for a max of 10 stacks. Each stack gives 2% Pyro DMG Bonus and lasts for 3 seconds.
Marks one enemy with Blaze which does an AoE explosion that can only be triggered by other party members. When an enemy marked with the blaze is defeated, it will pass on to a nearby enemy. The Blaze explosion becomes more effective when paired with an Anemo vacuum unit like Venti or Kazuha. As an explosion, it also deals reasonable blunt type poise damage, which can break shields. Note like Xingqiu's burst, Yoimiya's blaze does not snapshot but is instead dynamic.
With one of her passives, after using her burst, she gives the rest of the team 10-20% more ATK. This is basically a free Noblesse or Pyro Resonance buff (note it does not buff herself).
Her C6 has a 50% chance to fire another arrow that deals 60% of original damage. This effectively makes it easier to attain max stacks of pyro dmg bonus from her passive. It also increases expected/average dps by around $$1/2 + 1.6/2 = 1.3$$ times. This is a separate multiplier (akin to 50% crit rate, 160% crit damage).
Yoimiya has a number of options:
Here 2 piece Shimenawa can be replaced with 2 piece Gladiator for the ATK increase.
Aim for ATK% on the sands, Pyro DMG Bonus on the goblet, and CRIT on the circlet.
For substats, CRIT > ATK > ER.
Among 5 stars, Thundering Pulse is her best in slot. Skyward Harp is also decent for her. Polar Star provides good crit rate, but the other two weapons have more convenient passives for Yoimiya specifically. Among 4 stars, the Rust is her best option. The craftable Hamayumi and Prototype Crescent are also ok. Using the Compound Bow for physical Yoimiya is a meme but doable since she has high scaling on her autoattacks anyways.
As Yoimiya attacks fast with her NA, when paired with XQ/Yelan/Furina, she may not vape all her hits, and instead it might be the hydro subdps vaping. But this is similar to Noctis, who does not vape all his CA hits.
With Raiden or Chevreuse, her teams may proc the vapeload/overloaded reaction, and overloaded got buffed a bit in Version 1.6. Even with the overload buff, it may not make overload her best team. Other possible reactions include burgeon with Nahida/hydro. She can also try melt, but the game lacks a cryo XQ/Focalors. Perhaps Charlotte E is the current option as of 4.3 and maybe Kaeya/Rosaria Q to some extent.
Yoimiya can benefit from a shielder like Morax/Diona/Thoma/Layla and an Anemo unit like Venti or Kazuha will help Yoimiya even further.
As her best teams usually always have Bennett, her vv rotation can look like: Bennett E/Q, vv shred, hydro subdps E/Q, Yoimiya E, fire away.
As Yoimiya provides an ATK buff (giving her some support abilities) and can be used in a quick-swap manner, it is possible to pair her with Xiao, Raiden, XQ, Hu Tao, Tartaglia and function as a kind of sub-DPS/harmony support. It is viable in overworld, and people have cleared Abyss with such combos. This ATK buff may give her more synergy with XQ than say HP scalers like Yelan, but Yelan or Furina give large enough dmg bonus buffs to Yoimiya to offset this.
Here we assume talent levels 10-10-10. We assume Yoimiya has a Pyro DMG Bonus goblet and is running 4 piece Shimenawa (which gives 50% Damage Bonus), so her total DMG Bonus during her skill is at least \(96.6\)%. Within the 10s duration of her skill (which has an 18s cd), she can do her whole 7-hit NA combo around 3 times. At level 10, the total NA scaling is \(762.27\), which will have a multiplier buff of \(161.74\) from her skill. The DPS is then
This is higher than Ganyu, C6 Xiangling with vape, and Hu Tao without vape. From her passive she will have around 6-7 stacks in a given moment considering her 7-hit NA sequence takes ~3 seconds and each stack lasts 3 seconds. This will give her around 12-14% extra Pyro DMG Bonus on average. With C6 it becomes easier to gain the full 10 stacks for a total of 20% extra Pyro DMG Bonus.
To get a fairer comparison with Hu Tao with vape, we would have to consider Yoimiya's DPS with vaporize, overload, or vapeload. But without reactions, Yoimiya's NA DPS is already higher than Ganyu and Hu Tao, though less than Xiao.
If Yoimiya does 21 hits during her E, from ICD, suppose she vaporizes 7 of those hits (via Yelan/XQ). In reality, we may not know which of the 7 hits gets vaped, so let us do an approximation. The average scaling for the 7-hit combo is $$z_m = 762.27/7 = 108.9$$ Suppose equivalently Yoimiya does 21 hits with this avg scaling. Then melting 7 of those hits is akin to doing 28 hits with this avg scaling. In other words that is a 4/3 or around 33% increase in dps. $$4x/3 = 538.64$$ Vaping 7 of those hits is akin to doing 21+3.5=24.5 hits with avg scaling $$24.5x/21 \approx 1.167 x = 471.31$$
As of Version 2.4, Yoimiya got a big buff from Yun Jin, who can increase Yoimiya's damage by around 1.5 times. See Yun Jin for details. The main result is that with Yun Jin, Yoimiya's non-reaction dps can reach $$x = 608.3$$ One example team where Yoimiya can leverage all these buffs plus vape is Yoimiya, Yun Jin, Bennett, Yelan.
Note an R5 rust gives 80% NA dmg bonus, which will increase the mv above even more. An R5 Thundering Pulse can also give 80% NA dmg bonus while also giving 66% crit dmg. Let us factor in the 80% dmg bonus into the above calculation:
That increases the dps by more than 1.5 times. So having a simple f2p weapon that buffs her damage by a lot is one of Yoimiya's great strengths.
With melt, we multiply the above by 4/3 to get $$4x/3 = 757.8$$ This is actually really high and cements Yoimiya's status as the single-target queen of dps.
With vape, we have $$1.167x = 663.27612$$