If Illya is the holy grail, then what is Kuro?
I know it's just a light hearted loli fanservice show, but you really didn't pay attention did you? Okay Kuro is the grail. She for all intents and purposes, is the Ilya from FSN except if Ilya from FSN had been sealed at birth and been forced to watch someone else grow up as she took a backseat.
Prisma Ilya is just a regular little girl. The Grail knowledge and all that shit was sealed into Kuro and the body was set as a blank slate and allowed to grow up normally.
So Kuro is really closer to the FSN Ilya than Ilya is.
Hiroyama created a dual Illya implementation that incorporated all of the awesome of FSN Illya with none of the flaws and then proceeded to ruin everything in 3rei by trying to out-Nasu Nasu, which is like trying to piss into a sea of piss to raise the level. Fate/kaleid fin.