Ruan Mei (阮梅), released in 1.6, is a cryo harmony unit. Ruan (阮) corresponds to Nguyen in Vietnam. The Nguyen dynasty was known for overthrowing the Le dynasty during the Qing era.
Ruan's aunt is Arlice. This is almost as funny as Klee's mother being Alice?
Ruan allows the team to deal more gauge toughness damage. Similar to Silver Wolf, she has no parallel in Genshin. However, Ruan's kit is a bit easier to translate into Genshin. In Genshin, most attacks will deal 1A, 2B, 4C gauge units of damage, with some things like Nahida's E applying 1.5 units. In the source code, gauge is referred to as 'durability'.
So all Ruan Mei needs to do in Genshin is increase the gauge units of attacks by 1.5 times. So 1A becomes 1.5 units, 2B becomes 3 units, and 4C (Beidou Q) becomes 6 units (and 1.5 units would become 2.25 units). Such a support would be useful in shredding things like abyss, herald, and lector shields.
If that was all she did, ... but that's not all folks, she does more! Her E buffs all allies' damage by 32% at lv10, making her a universal dmg buffer. The buff lasts 3 turns, so she is also sp positive.
Moreover, her Q increases allies' all type RES penetration by 30% for 2 turns. Moreover, enemies with their weakness broken will be delayed based on Ruan's break effect and take damage based on her break dmg. Genshin does not have RES pen, but this is equivalent to 30% universal res shred, which is a powercreep of zhongli.
So her E and Q already make her quite good. In Genshin, her E could buff damage by 30-40% for all allies for say 15s (can be better than kazuha's uptime) with say 10s cd. Her Q could shred res by 30% for say 10s with say 15s cd (60 energy). This is basically like kazuha's kit (dmg buff and vv res shred) except she buffs all elements, not just autumn ones.
Her talent increases team members' speed by 11%. In Genshin this could be equivalent to more atk spd or cd reduction. Her passive regenerates 5 energy each turn (burst costs 130 energy, so would take 26 turns at most to fill). In Genshin this could be like Barbara's passive and regenerate X energy every second (for 60 energy, say 2-3 energy per second so she gets her burst back within 20-30s).
Her other passive increases her E dmg bonus buff based on her break effect (exceeding 120%), up to 36% more dmg. This means her E can give up to a 32+36=68% dmg bonus buff at lv10. And this is where she just straight up powercreeps kazuha and Furina. At C0, the most Furina gives is 75% dmg bonus, and that requires hp fluctuation and 300 fanfare points and a burst. But here Ruan can give it for free and instantly. Ruan would literally make Furina eat macaroni for days, so perhaps the Genshin storywriters want to talk to the current director of Star Rail.
So is HSR trying to tell us that the 3rd Fatui Harbinger is a stronger harmony buffer than Furina? Well Furina was indeed nearly assassinated by the 4th one...
Now break effect does not really exist in Genshin, so instead we could make Ruan scale off some other stat like EM or HP.
E1 (Neuronic Embroidery) makes allies ignore 20% def when Q is up. E2 gives 40% atk buff on weakness broken enemies (this is eh considering Navi has this as a C0 passive, like wth, but if you are trying to say Xiaogan is stronger than the 3rd Fatui Harbinger, then it makes total sense: bro come fight me). E4 increases break effect by 100% when enemy weakness is broken. E6 extend Q by 1 turn and increases break dmg multiplier.
E1 is good. As has been observed, def shred seems to be much more ample in HSR than in Genshin, considering you have units like Silver Wolf that can shred def with their god-forsaken normal attack.