The focus of a discarded plotline that would have run through volumes 2 and 3. She was conceived of as an unfortunate, mute little girl who also happened to be a personality of the Hundred Faced Hassan. However, due to amnesia even she would have no idea who she really was. It seems that while he was still alive, this personality was the one that would surface whenever the Hundred Faced Hassan was captured by the enemy and tortured for information, in order to protect his secrets.
Anyway, during Team Rider's raid on Gilles' workshop, she would manage to separate herself from the other personalities before they got slaughtered, after which Waver would mistake her for one of Ryuunosuke's victims and place her under his protection. Of course, Rider would recognize her as a Servant immediately, but keep quiet out of respect for Waver's chivalrous spirit, allowing the (fake) Mackenzie family to gain yet another member. Unfortunately, though, Kirei would be aware of all of this, and in the third volume would use his command spell to force the girl to remember her true identity as Hassan. The resulting tragedy would push Waver one more step down the path of adulthood.
…Well, that was the plan, but in the end I decided that it would just give people the wrong idea about who the protagonist of Fate/Zero was actually supposed to be. So, I swallowed my pride and shelved the whole thing. The only part I kept was the workshop raid scene, since I had already written it.

Fate/Zero material: Fate/Zero Encylopedia