Illya's Maids

The maids accompanying Illya.
The only reason they appeared at all is because the art director kept insisting that the story needed maids. Or rather, it was more like he threw a fit.
Sella was the one responsible for Illya's education, while Leysritt was her conversation partner. Incidentally, did you know that Leys actually has some of the biggest breasts in Fate/stay night?
Originally, they were failed attempts at creating a Holy Grail. They were scheduled for disposal, but ended up having their lives prolonged as the homunculi assigned to take care of Illya.
Also, this is just a digression, but at one point Illya's caretaker was going to be a wonderful suit-wearing German man who relentlessly teased Shirou and Rin.

Fate/side material: Fate Encyclopedia