Who to fodder Exalt Chrom to?

I'd say the ones that benefit the most from Chill Def are:

-Support units, like Marth, so they can grant even more support(also Marth benefits from it too).
-Anti-armor heroes and Brave weapon users. Those that aim to defeat an opponent by oneshotting(two-shotting in the case of a Brave weapon) them need as much damage potential as possible. Bartre, Frederick, Cherche, I'd say Elincia too but she probably benefits more from Hit and Run, or Desperation if she's built to quad.
-Low attack, but fast heroes can benefit a lot from it because it's technically a +14 to their damage. Karel/Fir, Lon'Qu, Clair/Sheeda(here Chill Defense helps a ton against both armors and non-armors since they have low attack but they also have anti-armor weapons).

You could also put it on someone that's going to be in the same team as Gunnthra, because she already comes with Attack, Speed and Resistance debuffs in her base kit.

Also, Aether is not bad on Cherche, it's just that she usually has enough attack already so she prefers Galeforce, or if she needs the extra damage she's probably better off with Moonbow/Glimmer.

Think about someone you want to build and consider Chill Defense is optimal on, that way it'll get more use.

I can give some advice:

Support units use it well, they can help support more. Two examples are Marth and Corrin.

Slow Brave Weapon users can take advantage of it as well, units like Cherche with high attack.

Units who can or try to one shot with it. Like Eliwood and Walhart.

Units who can do effective damage against their targets. The choice of character can vary for this one, but I have someone on my friend list who has a Kaze with that dragon-effective festival dagger with Death Blow and Chill Def.

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Also, Aether is almost never TRULY optimal IMO, it's 9 times out of 10 just a shoe-in for arena points.

Walhart is a limited unit so if you like chill def you might just use him as an extra chill def, which means no aether.