I have added a tier list to make this easier to consume, this tierlist is not to be taken as a rigid standards, depending on the units you have and on the eidolons as well as just personal interpretation, units on this tierlist would move a tier up or a tier down.

Also keep in mind team synergy and slots you have to fill, using Huo/Ting/Bronya with him is bad even though they are all S tier because he gets no debuffs, the team is missing a role. Same thing with Ting/Bronya/Ruan Mei, in this case you don’t have enough debuffs AND you don’t have a sustain unit.

Ideally, at E0, a proper team setup for him includes: Ratio, Sustain, Energy, Debuffer

Energy includes Huo/Ting/Bronya and is also the least important role to fill, in addition to that, Huo would be in the Sustain role and would free up this slot. The reason I recommend this role is because unless you use an ERR Rope, which will decrease your damage by a lot, or an energy battery, you are going to be locked into a 4T guaranteed cycle. Any of these mentioned factors will pull you ahead a turn into a 3T.

Debuffers are going to be the main source of applying debuffs for Ratio. In the new kit, Ratio requires a minimum of 3 debuffs, and he applies one on his own. That being said, I would recommend pairing him with a debuffer that can sustain 3 debuffs on their own. Ratio benefits from up to 5 debuffs an ephemeral debuff that can be covered by his technique. He only needs 6 debuffs on a single enemy once to get all the CV buffs, he requires 5 debuffs for the enemy to take his full 50% DMG buff, and he only needs 3 debuffs to be usable.

I am assuming all the 4* units are E6 and all the 5* ones are E0 including Ratio, I will try to mention the important eidolons for the 4s and 5s.

Topaz: at E0S0, she gives Ratio a pretty nice increase to damage and a perma debuff on the enemy, adding an E1 adds an extra debuff and more damage increase and the same with an S1. Note that her S1 can also be used directly on Ratio if you are not using her in the team and you do not have his own LC, it’s the second best LC for him and gives him a second debuff. Unless she is at E1S1, I would recommend using a debuffer with her, at E1S1, she still could benefit from having a debuffer alongside her.

Huo Huo: Energy regen. Ratio also really likes attack buffs so the fact she applies one to him is also important. She also covers the solo sustain slot. Overall makes building the team a lot easier.

Tingyun: Attack buff and Energy regen. You know Tingyun.

Bronya: Makeshift energy regen but does require a lot of SP. Her eidolons are all really helpful, but especially her E1 and E2, besides that, her S1 is also great for relieving a bit of that SP problem.

SW: Covers all the debuffs needed, allows brute forcing and makes the team much easier to build.

Ruan Mei: Even though she gives DMG% buffs, those in addition to the PEN, as well as the break efficiency stuff which also means you get non-horrible uptime on an extra debuff especially since Ratio breaks a lot and being imprison means that it’ll stay for even longer. I would not say that this replaces the basic need for a debuffer, but it could def help with the extra DMG% if you don’t have 5 debuffs in the team already.

Guin: E1 Pearls = 4 debuffs Ratio talent = 5 debuffs, alongside that, she also gives a bit of DMG% and ER down without using too much SP. The best unit to solo debuff after SW.

Luocha: solo sustain and same element, which makes it easier to use SW, you can use Penacony set as well, and could make Wastelander easier to use. More SP than Fu Xuan.

Fu Xuan: solo sustain and CR buff. Also allows for SW weakness debuff.

Yukong: huge ATK buff that is actually usable with his new kit. You can also tailor his stats to use with her but you could also use her to cover his weaknesses. Possible Planetary LC and Penacony set. At E6, you can probably have some interesting interactions between her ult and his ult.

Asta: SPD and ATK buff. Has a debuff (I have a bit of EHR on Asta because I usually run her with DoT team, this works really well here as well. Does require fire weakness if she isn’t E6 to conserve SP or even keep the buff up. Best used with Topaz.

Pela: if you don’t need a full range of debuffs, she is really great, but she requires E4 and Pearls as well as a lot of SP and ults to keep it rolling at 3 debuffs, the EHR buff is useful for Ratio and Asta if they are in the team.

Welt: 3 debuffs without Pearls, 2 locked behind his ult. He also loses a debuff if you break the enemies and I am not sure if his and Ratio’s techniques conflict. He can make Ratio use Wastelander more comfortably, but also removes the “extra debuff on broken enemy”

Hanya: strictly SP , ATK buff, and SPD buff, if you can keep the ult up on Ratio all the time, she is super powerful, but at the same time, I think there are better choices. She is pretty good, I just think Asta usually does better with Ratio in my opinion.

Gepard: normal use, pretty SP could swap for an extra debuff if needed at E1. Usually can solo sustain. at E2, with Markets LC, might be able to apply 3 debuffs on the enemies as a sustain, not recommended, but he is the only solo sustain who can do it so worth mentioning. I want to test this further.

Luka: debuffs and extra damage, he can also hold his own damage and likes having his break on the enemies, with Pearls, he can have 3 debuffs, and his break is a 4th, damage received debuff and Pearls add a bit of damage to Ratio, I believe he might be a good solo debuffer with RM in the team.

I don’t want to discuss the lower tiers, if there is a specific unit there you want to ask about, I would gladly answer.