presence concealment

Assassin has the special ability of "Presence Concealment" that other Classes do not possess. He is unrivaled in concealed movement and tracking.

To Kirei, whose responsibility is not to pursue victory but to provide backup for Tokiomi, summoning Assassin was the best choice.

That is the battle plan.

First, let Kirei's Assassin go around thoroughly investigating the battle stratagem of other Masters and their Servants' weaknesses. After obtaining a method of certain victory against each enemy in this manner, Tokiomi's Servant will then be used to defeat each one accordingly.

- Fate/Zero: Act 1, Part 3