Hisau Maiya

Emiya Kiritsugu's assistant. Also his lover, in a certain sense of the word.
Due to growing up utterly deprived of humanity, Maiya lacks a strong sense of self, and has given her entire being to Emiya Kiritsugu. Asking her if she loves him would be the same as asking the internal organs if they love the brain—completely meaningless.
And, regardless of what others might say, for Kiritsugu sacrificing his wife to complete the Holy Grail was a betrayal of their love. However, it was a betrayal which he absolutely must not hesitate to perform. His sexual relationship with Maiya, then, was simply a rehearsal for this betrayal—a form of masochism to calm his nerves. This is already pretty much the worst reason for having an affair, but the fact that Maiya valued it makes it an endless vicious cycle. Shirou, whatever you do, never ask your old man for advice on women…
Though her skill at sorcery is only a little better than Kara no Kyoukai's Azaka, she is a professional assassin capable of using any tool to kill no matter how crude it may be. So in that sense, she is a user of “deadly sorcery.” If push came to shove, she would be able to kill somebody with nothing more than a simple wooden skewer from a yakitori restaurant.
By the way, this is just a digression but Maiya has an absolutely devastating sweet tooth. There's actually an urban legend passed around among store clerks about a mysterious woman who would always show up at cake buffets all alone and start gulping down desserts one after the other with a sour look on her face. Of course, Maiya herself would die of shame if anybody ever found out about this.

Fate/Zero material: Fate/Zero Encylopedia