I have no idea about the rate, but I made Morta drop 5 SRs in 94 kills, and Nona dropped herself 4 times in 200 kills, with one or two random SRs there. Regea also dropped two of herself, which is UR, on the first 15 kills, but now I'm at 100 kills and she hasn't dropped again.
It's pretty much 'RNG pls'.