Yun Jin (云堇) is a Geo polearm character more useful than Zhongli.
Her damage and buffs scale with her DEF. Her E is a counter like Beidou's (except when Yun Jin holds it long enough it will guarantee charge level 2 parry). Talent priority is Q > E.
All her constellations are quite good. Her C2, C3, C4, and C6 are the key ones that boost her support abilities.
Husk (released in Version 2.3) 4 piece is her bis for boosting DEF and Geo DMG.
DEF/ER sands, DEF/Geo DMG goblet, DEF/CRIT circlet. Substats: DEF, CRIT > ER. Yun Jin ascends with ER.
5 stars: Homa, PJWS, Skyward Spine. 4 stars: Catch, Favonius Lance, Deathmatch.
She works well with Yoimiya and Eula. She can also support Diluc, Childe, Itto, Raiden (though these characters may also rely on their skill or charged attacks for damage, not just normal attacks). If you use Xiao's normal attacks (Xiao has highest scaling on normals among polearm users), Yun Jin can help boost those as well. She can also work with Razor, Kokomi, etc.
The buff from Yun Jin scales with her DEF. At Lv13 Q and assuming there are at least 3 different elements in her team (which gives another 7.5% DEF scaling) the total bonus is \(68.34+7.5 = 75.84\). As in Shenhe's case, the effect of Yun Jin's buff on damage is $$1 + \frac{bonus}{ATK*scaling} = 1 + \frac{.7584(\text{Yun Jin DEF})}{\text{Character ATK}*scaling}$$ The buff is bigger for attacks broken into parts with smaller scaling, such as the normal attacks of Yoimiya. If the scaling is 100%, Yun Jin's DEF is 2000, and the character's ATK is 2000, then the buff is about 1.7584 times more damage. In Yoimiya's case, the scaling on her normals on average is within the range of 100%, so Yun Jin can basically multiply Yoimiya's damage by 1.76 times, which is more broken than the buff provided by Bennett (about 1.5 times) and simply knocks Zhongli out of the park (about 1.17 times from RES shred).
The caveat is that it only applies to normal attacks (not even charged or plunging), whereas Bennett's ATK buff applies to everything. But Yun Jin's buff is a bit more applicable than Shenhe's buff (which is only for Cryo, but for all types of attacks). So Bennett is still a top tier support and top 1 or 2 buffers in the game. But the fact that Yun Jin's buff is comparable to the mighty Bennett's, even exceeding Bennett's buff in certain situations, reflects highly of Ms. Yun.
Both characters have an energy cost of 60 and scale with ER. Although Bennett can heal and does not have a quota, his circle is fixed in space, whereas Yun Jin's flag buff follows the character around, wherever they go, and she can create crystallize shields. Bennett and Yun Jin both buff the character and provide some defensive value, but they do it in different ways.
So why not use both?
Let's say we use both characters (on say a Yoimiya-Bennett-Yun Jin team). To simplify we ignore the effects of Pyro resonance and assume that Bennett gives 1000 ATK to Yoimiya, bringing Yoimiya's ATK from 2000 to 3000. This ATK increase multiplies damage by 1.5. Then Yun Jin's buff will increase damage by $$1 + \frac{.7584(2000)}{3000*1} = 1.5056$$ which is basically another 1.5 times multiplied to Bennett's buff. In total Yoimiya gets a buff of $$1.5 * 1.5056 = 2.2584$$ so Yun Jin and Bennett together can double Yoimiya's damage on average, which is quite broken. This is more than melting every shot. That's great, but most characters can use Bennett, so we will remove the 1.5 buff from Bennett as it can apply to most other characters. Yoimiya still gets a 1.5 times buff from Yun Jin (after accounting for Bennett) which is equivalent to vaporizing every shot without the ICD restriction. So Yun Jin is basically Yoimiya's Xingqiu or Hydro support.
Previously Yoimiya had a dps of around 404. With Yun Jin's buff this increases to around $$x = 608.2624$$ This puts C0 Yoimiya above C1 Hu Tao. So in retrospect, when the 2.4 update released Yun Jin and Shenhe, they were basically customized buffs to Yoimiya and Ayaka.
Note the above is based on a average analysis, assuming that the average scaling of Yoimiya's normal attacks is 100%. The actual average is \(762.7/7 \approx 108.96\) which is slightly more but does not affect the final result too much. For an exact result, the effect of Yun Jin's buff needs to be calculated for each separate hit in Yoimiya's 7-hit sequence. For hits less than 100% the buff will be greater, and for hits more than 100% the buff will be less. 4 of the 7 hits are less than 100% whereas 3 of the 7 are above 100%. The average only gives a rough estimate.
This damage does not factor in reactions. Nor does it consider the boosts from Yun Jin's C2 (+15% NA damage after Q) and C6 (+12% normal attack speed) for Yoimiya. The team consists of Yoimiya-Yun Jin-Bennett. The 4th slot can be Raiden, Fischl (for Overload) or Xingqiu.
Lastly, the trigger quota of 30 hits is usually more than enough for the relevant characters. For example, Yoimiya can do 21 hits (3 times her 7-hit combo) within the duration of her E. And both Yun Jin and Bennett's buffs last 12s, long enough for the duration of Yoimiya's E.
Quicken (released in 3.0) is a reaction that is also centered on the additive buff. Due to icd, not every hit may proc spread/aggravate. As addition is commutative, it does not matter which hit gets the additive buff, whereas for multiplicative buffs, one generally wants to vape/melt on hits that have higher scaling to deal more damage. The goal of additive buffs is simply to proc it as many times as possible (it does not matter as much which hits proc it).
If Yun Jin has 2k def the flat damage bonus is $$b = .7584*2000 = 1516.8$$ which has effective scaling of around 50% (divide by 3k atk).
For Yun Jin's quota of 30, the max flat damage one can get is (if she has 2k def) $$30b = 45504$$ The effective scaling (normalizing for 3k atk with Bennett) is around 1500%. So Yun Jin's buff can add around 1500% scaling to NA attacks, which is quite a lot. Yun Jin's q has a cd of 15s so the extra mv is then around $$1500/15 = 100\%$$ If Yoimiya uses 4p shimenawa for 96.6% dmg bonus the additive increase in dps is $$100*1.966 = 196.6\%$$ So Yoimiya gets nearly 200% more mv.
Some claim that Yun Jin dragonstrike with C6 Bennett, XY, Furina can clear abyss. We did some calculations in plunging, and the mv is around 700 for such a team (for basically most melee characters). This is indeed high enough for clearing abyss floor 12.