Depends on what culture you want to draw from. If you're familiar with Chinese or Japanese history (or possibly Chinese wuxia) there are quite a few good possible Assassins to be found from those, depending on how far back you want to go (like Jing Ke and Zhuan Zhu). Roman assassins might include Marcus Junius Brutus (or Cassius, if you prefer), or Marcia, the mistress of the emperor Commodus. More modern figures might include Guy Fawkes (or one of the Hashashin, which you could make up a completely different Zabaniya for) or even someone from the 19th or 20th century.
(Because of the nature of Assassins, most of them are only Heroic Spirit Candidates before being summoned and are thus essentially wraiths, possessing lower parameters than a true Heroic Spirit on average. Essentially - assassins had to have been human to have been able to assassinate someone ... sad but true.)