I do beg to differ. That one is no masochist, while Tenshi most definitely is. And conisdering the inability of repeated electrocutions to teach Kuroko to not try molesting her onee-sama, I think it's safe to say that Kuroko either (A) is a masochist (B)has a lower ability to learn than the average dog or, indeed, cockroach.
Dalveska said: If ya go by ability...wouldn't Yukari fit best for Kuroko?
That would only work if Biri-biri is Reimu. I say Aya would be a better choice since her super speed could be like teleporting, both require work so it hardly matter.