I'm late to the party, but I have to say I agree with Goldva. I never bothered to use any other familiar after unlocking her, even though I'm sure the others are stronger, just the fact that she can be a heal backup as well as cutely perching on your shoulder when you're idle/sat down is just the best. Also, playing the piano in the couryard with her is exactly what an action game like Bloodstained needed.