I would write a whole wall of text to explain what I think about the supports for Ratio, but I feel like people wouldn't read that, so instead, I am throwing this up so people can digest the information in a more palatable manner.

Let me explain some of the choices.

With the current kit, Ratio requires 3 outside debuffs to fully function. He will probably either build hybrid DMG% sets or 4-Wastelander. I did not count the chance of him using Topaz LC, that will give him an extra debuff but decrease his damage but at the same time opens up the team options. The only other LCs he could be using are Only Silence, Stellar Sea, or obviously, his signature. Those are my assumptions for this listing.

I rated units based on their roles, so Hanya is a bit higher because she is a Harmony who can apply a debuff, same with Asta for example. Debuffers are rated lower if they do not apply enough debuffs or they need ults, but keep in mind you can use Pearls on Nihility to boost them up into the spot in your team, but even then throwing Pearls on a unit with a lot of debuffs at no SP cost vs. someone who needs ult and SP to apply them keeps them in the tiers I have.

Topaz, the more you invested into Topaz the better she is for Ratio, the better your Ratio is the better your Topaz becomes, they build on eachother pretty well, and she can apply 2 undispellable perma debuffs and 1 dispellable perma debuff on the enemy at E1S1.

SW, so many debuffs and a lot of damage increase, if you run her against enemies weak to IMG she ends up giving herself toughness break and can work with either of the sustain twins without changing anything in the elemental balance.

Energy regen, his ult cost is 150 and he wants it every 3 turns, even then you are banking on a dead turn. His ult usually comes in on 4 turn rotations at E<4 which is why energy regen units good Tingyun gives 50 which guarantees 3 turn roto without caveats but Huo covers sustain and realistically could usually still hit 3 turn without ERR Rope

Asta, if the enemy has a fire weakness or if you are running with E0S1 or E1S0 Topaz, she covers the final debuff while giving you the speed and attack buffs for both of them. She might be higher, but I decided to lower her a tier because she really wants the fire weakness and ideally the Topaz.

Guinaifen, comfy number of debuffs at 4 with E1 and Pearls. Firekiss increases your damage and her own E1 debuff allows his ER debuff to land more easily. Helps out a bit with AoE damage. Does not need ult or SP often. Reason she is not higher is because her damage amp for Ratio is on the lower end.

Bronya, she might need to be higher because she allows more damage and faster ult regen but I put her in A because she is SP-1.

B-tier, it’s all debuffers that have a difficult time keeping their debuffs/don’t have enough debuffs without Pearl and require resources to get them. I would put Welt a tier higher in case you are using 4-Wastelander set on Ratio.

Gepard at E2 with the Topaz LC actually has 3 debuffs with a bit of SP investment, that being said, his speed debuff comes after the freeze so you will usually have 2 up with that LC. You need an E2 and you might have issues sustaining since you don’t have the draw LCs on him. BUT in a pinch? I feel like he could come in clutch.

Yukong, I don’t think she can keep up with Ratio and I don’t know if the Penacony set with Planar would help her. Even at E6, his FuA from ult is too RNG and the only place that I think she might work is if you have some Topaz setup that frontloads the damage into Ratio’s but after the first turn it will probably break apart.

If you have questions about specific units, you can ask, but this tierlist isn't rigid at all and blah blah usual disclaimer