Irisviel von Einzbern

One of the many homunculi modeled after Justeaze Lizrich von Einzbern.
Normally, Einzbern homunculi are created with a specific purpose in mind, and their bodies and minds are optimized for that purpose from birth. However, because the plan to conceive Illyasviel was only proposed after Irisviel had already been brought into the world, a special exception was made and she was allowed to take on a second role in addition to her original. Namely, she was charged with giving birth to and raising Illyasviel. Perhaps Jubstacheit determined that this was more efficient than crafting a whole new homunculus to act as a wet nurse, even if it flew in the face of normal Einzbern practices. At any rate, the end result was that despite being a mere homunculus, Irisviel was able to experience the joy of motherhood just like a human, which in turn affected her sensibilities in ways that Old Man Acht1 could not fully anticipate.
Amongst the Einzbern homunculi, there had never been a specimen that spent such a long time interacting with humans, or that had such a colorful and irregular personality. Or to put it another way, a homunculus like Irisviel that was treated equal to a human and even possessed human emotions, was a first for the Einzberns. Because she possesses naturally superior mental and physical faculties, she was able to mature rapidly, eventually attaining self-consciousness and feelings on par with a normal human's.
Nevertheless, all Einzbern homunculi are burdened with inborn knowledge and reason, the crystallization of a millennium of wisdom. Compared to that, a mere nine years of emotional development and life experience is nothing more than a drop in the bucket. As a result, Irisviel ended up becoming quite the troublesome princess, possessing both the refined grace of a lady and the naivety of a child in equal measure.
By the way, back when I was putting the final touches on Zero's plot, I still hadn't heard any of the new setting information revealed about Ahriman2 in Hollow Ataraxia. When I think back about it now, the fact that I was able to arrive at “that ending” anyway is almost scary.

Fate/Zero material: Fate/Zero Encylopedia
Translator's Notes
  1. ^ Jubstacheit von Einzbern, the current head of the Einzbern family.
  2. ^ Angra Mainyu.