It's known that Aether is better than RFK from about 22 SKL until "very high levels". I disagree with the latter. At the point where RFK can even raise Luna to 100%, Aether's output is still more valuable than RFK ever will be outside of Streetpass (where RFK can be used to boost Lethality).
When I said, "Very High Skill" I mean 90.
AKA: 100% Luna.
Two units:
[Note: +SKL/-DEF]
Chrom!Inigo!Morgan
Assassin, Mystletainn
Limit Breaker, Aggressor, Rightful King, Luna [Note, could also be Ignis. Same output], Galeforce
Rallied, Tonic'd, (S-rank) Hero Pair Up
Olivia!Lucina!Morgan
Assassin, Mystletainn
Limit Breaker, Aether, Ignis, Luna, Galeforce
Rallied, Tonic'd,(S-rank) Hero Pair Up
Total Stats (which is equal due to common parentage, class and weapon.) 54+15 41+15 65+25 59+20 56+15 39+19 38+15 Proc Chances F-Morg: 45% Aether 49.5% Luna 4.95% Ignis 0.55% Nothing Proc Chances M-Morg: 100% Luna Attack vs 55 DEF/Dragonskin (Anna's stats, Generic target [no Weapon Triangle involved], Weapon Rank bonus not included): 14
Aether Proc: 14 + 27: 41 Luna Proc: 27 [33 Agg Phase for Male Morgan] Ignis Proc: 27 33.2285 Average: F-Morgan 30 Average: M-Morgan Secondary 100% PP engage (100% Aggressor vs 50% Aggressor): 33 Average: M-Morgan
Even if every attack made by Male-Luna+-Morgan is during Aggressor, Female-A/L/I-Morgan is still better.
Aether>RFK at all times once any reasonable amount of SKL is obtained. I rest my case.