Natalia Kaminski

A lone-wolf magus hunter. She had no affiliation with the Association, and made her living by swooping in ahead of the Association's enforcers, poaching their prey, and then reselling the spoils at an inflated price.
Despite their rather grandiose official title of “sealing designation enforcers,” genuine sealing designations are actually extremely rare, and most enforcers would consider themselves lucky if they got a chance to pursue such “big game” even once in a decade. Especially in the case of outside freelancers, they spend most of their time hunting heretical magi that have simply deviated from the Association's norms.
In the distant past, Natalia's ancestors crossbred with succubi, and while she doesn't have eternal youth or regenerative abilities, she does boast inhuman reflexes and coordination, as well as the special ability to boost her magical energy stores by consuming semen.
Although she is a money-grubber willing to do anything to make a quick buck, on top of being a wild hedonist that has never been a dollar ahead in her life, she somehow ended up as the morally questionable guardian of an impressionable youth. And while she sexually harassed the young Kiritsugu on numerous occasions, she never once drank his semen. The reason being that she had really fallen in love with him, if only a little—though like a good older sister she locked away these girlish thoughts and never revealed them to anybody.
By the way, her heart-rending and gruesome end was inspired by the movie Snakes on a Plane, which I saw with Nasu Kinoko.

Fate/Zero material: Fate/Zero Encylopedia